Showing papers in "Spe Formation Evaluation in 1989"

Use of Pressure Derivative in Well Test Interpretation

Abstract: In this paper, a well test interpretation method based on the analysis of the time rate of change of pressure, together with the actual pressure response, is discussed. A differentiation algorithm is proposed and several field examples are provided to illustrate how the method simplifies the analysis process. Interpretation of well tests is therefore easier and more accurate.

Resolving Pore-Space Characteristics by Rate-Controlled Porosimetry

Abstract: By monitoring the mercury capillary pressure in rate-controlled porosimetry (intrusion) experiments, new information regarding the pore space of a rock sample has been obtained. With this technique, called an apparatus for pore examination (APEX), it is now possible to resolve the pore space of a rock sample into two interconnected parts. One part identifies the individual pore systems (pore bodies), which are low-energy sumps or regions of low capillarity. The other part corresponds to the pore throats that interconnect with pore systems. New capillary-pressure curves have been obtained by partitioning the total capillary-pressure curve (normal capillary-pressure curve) into two subcurves: the subison capillary-pressure curve, which details the distribution of pore bodies, and the rison capillary-pressure curve, which details the distribution of pore throats. The authors present APEX data on Berea sandstone and San Andres dolomite that show the volume distribution of low-capillarity regions within the pore space of these rocks. These regions of low capillarity are the principal pore-space regions that trap the residual nonwetting phase upon imbibition of a strongly wetting fluid, as measured by toluene/air systems. The residual nonwetting-phase saturations as determined by the APEX method and by the toluene/air method are in excellent agreement. Thus, the detailedmore » volume distribution of pore systems responsible for trapped nonwetting-phase saturation is determined from APEX measurements, which can have important implications for EOR.« less

Geochemical logging with spectrometry tools

Abstract: Description d'un outil de train de tige, le GLT, qui mesure les rayonnements gamma de capture des neutrons (naturels, actives et rapides) et permet de determiner les fractions ponderales de divers elements (Si, AL, Ca, Fe, S, Ti, Gd, K, Th et U) afin d'arriver a une bonne description geochimique de la formation. Cette methode, ameliorant la connaissance mineralogique du fond de trou, devrait permettre de faciliter l'interpretation d'autres diagraphies affectees par la mineralogie

Calculating effective absolute permeability in sandstone/shale sequences

Abstract: In this paper two averaging algorithms are proposed for determining block effective absolute permeability. The experimental relationship between the effective permeability, the volume fraction of shale, and the anisotropy of the shales is first observed through repeated flow simulations. A power-averaging model and a percolation model are proposed to fit the experimentally observed relationship. The power-averaging model provides a surprisingly easy and efficient way to calculate block effective absolute permeability. A simple graph is given to determine the averaging power from the geometric anisotropy (aspect ratio) of the shales for both vertical and horizontal steady-state flow. A correction for large shales relative to small gridblocks is also proposed.

Horizontal-well pressure analysis

Abstract: This paper presents an analysis of the pressure-transient behavior of a horizontal well or a drainhole. The performances of horizontal wells and fully penetrating vertical fractures are compared. Dimensionless wellbore pressures are computed for two classic boundary conditions: infinite conductivity and uniform flux. Results are presented as pseudoskin factors and as type curves. In addition to conventional pressure-vs.-time type curves, derivative type curves from pressure/time predictions are presented. The derivative approach the authors discuss is applicable to a broader range of problems than considered here.

Effect of overburden pressure and the nature and microscopic distribution of fluids on electrical properties of rock samples

Abstract: Laboratory equipment aimed at determining the exact correlation between resistivity and water saturation under stress, pressure, and temperature conditions is described in the first part of this paper. The porous-plate method adapted to reservoir conditions is used to obtain different saturation values during both drainage and imbition. With this equipment, the influence of the effective stress on the porosity and formation resistivity factor can be studied before the test. In the second part of this paper, the values of the formation resistivity factor and resistivity index are compared for water-wet samples from sandstone and carbonate reservoirs. These measurements indicate that the influence of the effective stress depends on the nature of the rock sample. In addition, the resistivity/water-saturation law depends on the direction of the saturation change (drainage or imbition) and on the nature of the fluids (water/oil or water/gas).

Relationship between the Archie saturation exponent and wettability

Abstract: Les imperatifs de faisabilite en recuperation assistee exigent une connaissance de plus en plus precise des saturations en huile in situ. L'amelioration des interpretations quantitatives des logs de resistivite necessite d'une part d'etablir une relation generale entre l'exposant, n, de l'equation de saturation d'Archie et la mouillabilite, et, d'autre part de pouvoir determiner in situ la mouillabilite par logging de la pression de la formation. Cet article decrit une relation lineaire entre l'exposant de la relation de saturation d'Archie et la mouillabilite des huiles. La pente de la droite representatrice depend des caracteristiques petrophysiques de la Roche; elle est identique, pour une meme roche, a basse et haute temperature. L'exposant n passe de 2 a 8 pour les roches mouillees a l'eau ou a l'huile. la mouillabilite a l'eau des systemes huile/eau/roche augmente avec la temperature

Elastic and Viscoelastic Calculations of Stresses in Sedimentary Basins

Abstract: This study presents a method for estimating the stress state within reservoirs at depth with a time-history approach for both elastic and viscoelastic rock behavior. The model can incorporate changes in pore pressure, temperature gradients, consolidation and diagenesis through time-varying material properties, and varying tectonic episodes. Example calculations are presented for simple cases and for a complete stress history of rocks in the Piceance basin.

Two-phase pressure test analysis

Abstract: A theoretical basis is given for well test analysis of solution-gas and gas-condensate reservoirs in the infinite-acting period. The study is limited to radial flow with a fully penetrating well in the center of the drainage area. Porosity and absolute permeability are constants and gravitational and capillary effects are neglected. The tests are conducted at constant surface rate. An analytical expression for the pressure - saturation relationship is derived from the time-dependent gas- and oil-flow equations. This relationship can be used at the wellbore to generate pseudopressure functions for drawdown and buildup that allow test interpretation using the liquid analogy. Theoretical developments are examplified by simulated drawdown and buildup tests in a solution-gas drive reservoir. 7 refs.

A mathematical model for pressure evaluation in an infinite-conductivity horizontal well

Abstract: A mathematical model was developed to study the transient pressure behavior in a well with an infinite-conductivity horizontal drainhole in an infinite slab reservoir. The physical model includes a fluid of small and constant compressibility flowing through an infinitely large anisotropic reservoir with upper and lower impermeable boundaries. The analytical solution is obtained by applying the concepts of instantaneous sources and Green's functions. The authors discuss the uniform-flux model, a special case of the infinite-conductivity model, and present a simple way to use it to calculate the pressure at the wellbore face. They suggest that the pressure for the infinite-conductivity case can be evaluated with the uniform flux model at a fixed point along the wellbore with an error of less than 1% for combinations of the various parameters that may be encountered in real situations. They show to what extent the accuracy of the model will be affected by neglecting gravitational effects.

Effects of a partially communicating fault in a composite reservoir on transient pressure testing

Abstract: Many reservoirs are faulted, and hydraulic characterization of these faults for the design of field-scale developments. In addition to the effect that a fault has on fluid flow in the reservoir, it may separate two different reservoir regions with dinstinctly different properties. The detection of the properties on both sides of the fault and the distance to the fault are important in the reservoir-characterization process. In this study, a linear fault is modeled as an infinitesimal-thickness skin boundary. Analytical solutions for presure-transient behavior for a line-source, constant-rate well in a composite-reservoir are obtained with one Fourier space transformation and time-space Laplace transformations. The solutions are presented for strip and infinite reservoirs. This study examines drawdown-pressure and pressure-derivative behavior and extends and generalizes many studies presented in the past. Correlating parameters for pressure-transient responses are presented. The possibilities of boundary detections are considered, and interference pressure responses in a composite, strip reservoir are briefly discussed.

Relative Permeability From Unsteady-State Displacements With Capillary Pressure Included

Abstract: A semianalytic method for calculation of relative permeabilities from unsteady-state displacement data is developed. The method is based on the simultaneous solution of the fractional-flow equation and an integro-differential equation, derived from the material-balance and Darcy equations for immiscible displacement of two incompressible phases, including the capillary pressure by use of various volumetric relationships. This method for calculating two-phase relative permeabilities from unsteady-state displacement data is not restricted to the high-flow-rate experimental conditions used to overcome capillary and effects. Removal of this restriction allows the analysis of low-permeability cores with water and oil where the flow rates are low and capillary end effects are important. Our method allows direct calculation from the experimental data without intermediate interpretations of graphs.

Heterogeneities and geometry of sedimentary bodies in a fluvio-deltaic reservoir

Abstract: This paper gives the initial results of a two- and three-dimensional (2D and 3D) geological study that aimed at describing reservoir heterogeneities. This study serves as a basis for the conditional simulation of the geometry of a fluvio-deltaic reservoir presented in a companion paper. These two studies should solve the difficult problems of interpolating between wells and thereby the problem of forecasting the extension of sedimentary bodies. The research is being carried out by Inst. Francais du Petrole and Center of Geostatistics, Paris School of Mines. The study was made in the middle Jurassic in Yorkshore, which is very similar to the Brent formation in the North Sea. In the 2D phase, an outcrop consisting of a 1-km (0.6-mile) -long and approximately 30-m (100-ft) -high cliff was described with a photogeological interpretation. The sedimentological representation was carried out by means of several vertical cross sections. In the 3D phase, 36 boreholes were continuously cored and logged behind the cliff over an area of 1 km/sup 2/ (0.4 sq miles). The boreholes were about 300 m (985 ft) apart except in one grid where the distance was progressively reduced to 20 m (66 ft). This phase was designed to follow themore » evolution of the sedimentary bodies and their physical properties and to compare the correlation that could have been made (1) with fewer boreholes and (2) without the existence of the cliff. These studies provide new tools for understanding and describing the shape and heterogeneities of a reservoir.« less

Interpretation of Pressure-Buildup Responses in Gas-Condensate Wells

Abstract: Methods to analyze buildup responses in gas-condensate reservoirs producing below the dewpoint pressure are examined Two analogs to compute formation flow capacity and average reservoir pressure are presented These analogs are based on the steady-state theory (developed by O'Dell and Miller and later modified by Fussell) and the single-phase-flow theory (which ignores the existence of the two phases) The mechanical skin factor can be computed from the derivative response by computing the appropriate dimensionless pseudopressure

Earth stress measurements in the South Belridge oil field, Kern County, California

Abstract: The authors report earth stress magnitudes in the South Belridge oil field, determined from integrated density logs and microhydraulic fracturing test. They indicate that the vertical stress is generally the intermediate principal stress, except possibly at the deepest zone tested (2,100 ft (640 m)), where it is approximately equal to the lesser compressive horizontal stress. Azimuth of the greater horizontal stress and of induced hydraulic fractures, as measured or inferred by several different techniques, is N15{sup 0}E+-15{sup 0}.

Gas Storage and Transport in Devonian Shales

Abstract: Le stockage du gaz naturel dans les shales devoniens resulte en partie de l'adsorption du methane a la surface interne des pores. Lors de l'exploitation, le methane desorbe constitue une partie importante du gaz produit. La porosite, comprise comme mesure de la capacite de stockage du gaz, doit etre completee par un terme prenant en compte la phase methane condensee. En effet, l'usage de la porosite a l'helium pour estimer le contenu en gaz des shales, conduit a une sous-estimation de la capacite d'un facteur 2 ou meme plus. L'adsorption devient importante quand la taille des pores est faible et le rapport surface/volume eleve, comme c'est le cas dans ces shales. Ainsi, la desorption du gaz des murs du pore doit etre prise en compte dans les calculs de production. Les porosites et permeabilites standards ne sont donc plus des outils fiables pour la simulation numerique, lorsque l'effet de desorption devient important dans le reservoir. Le meme type de probleme se pose pour les phenomenes de migration du gaz dans le shale

Well-Test Analysis for Multiphase Flow

Abstract: Article de synthese sur les methodes d'analyse des essais de puits, notamment des donnees de pression relatives aux reservoirs exploites par poussee de gaz et des donnees de chute de pression

Simulation of Pressure Buildup in a Multiphase Wellbore/Reservoir System

Abstract: A pressure-buildup test is simulated in a system consisting of a wellbore and a reservoir and containing multiphase fluid. Wellbore flow is governed by conservation of matter and momentum, and reservoir flow by conservation of matter and Darcy's law. Finite-difference versions of the flow equations are solved on a discretized system. A single-phase example illustrates wellbore storage and wellbore fluid inertia shortly after shut-in. Two-phase examples illustrate phase redistribution, and one shows a ''hump'' caused by phases segregating. Buildup and other data from a field case are also matched.

Low-resistivity hydrocarbon-bearing sand reservoirs

Abstract: An example of water-free hydrocarbon production from formations that have water saturations >50% is discussed. It was demonstrated that relatively small amounts of smectite have sufficient surface areas to bind significant amounts of water. Whatever formations with large surface areas are present, high irreducible water saturations will result, Mixed-layer expandable clays are the source of the surface areas.

Effect of Drilling Fluids on Rock Surface Properties

Abstract: This paper discusses the wettability change induced by contact between porous media and drilling fluids and the possibility of eliminating such alterations by cleaning. Three porous media were studied (sandstone, shaly sandstone, and carbonate), as well as various drilling fluids (oil- and water-based). Initially strongly water-wet (hydrophilic) and initially neutral rock/oil/brine systems were evaluated. Wettability was estimated by a test based on spontaneous and forced displacement experiments. The results show that all the oil-based drilling fluids used induce a wettability change inasmuch as the rock is initially water-wet. The rock surface properties are observed to be affected at a distance >0.6 in (>1.5 cm) from the rock/drilling-fluid interface. Cleaning procedures with toluene and methanol circulations can return the rock surface to the original wettability state, but the permeability remains affected. Water-based drilling fluids made no appreciable change in the wettability of the three initially highly hydrophilic porous media.

New Pressure-Transient Analysis Model for Dual-Porosity Reservoirs

Abstract: This paper presents a new analytical model for interpreting pressure-transient tests for wells producing from dual-porosity reservoirs. This model includes unsteady-state matrix flow and incorporates the effects of wellbore storage, skin, and, for gas reservoirs, desorption. The model is applicable to bounded and infinite-acting reservoirs.

Influence of Microporosity on the Evaluation of Hydrocarbon Saturation

Abstract: Utilisation d'un modele 2D de pores preferentiellement mouillables a l'eau. La tortuosite electrique et la conduction de surface influencent l'indice de resistivite. L'exposant de saturation (pente obtenue sur le graphe log log indice de resistivite/saturation en eau) varie sensiblement en fonction de la saturation en eau. On peut donc en conclure que la technique commune de mesure de l'indice de resistivite a partir de quelques valeurs de saturation est insuffisante pour decrire une formation presentant une microporosite. L'obtention de valeurs acceptables de l'exposant de saturation n'est possible en laboratoire que par une experimentation systematique, les valeurs de l'exposant de saturation etant determinees au-dela de la gamme de saturation du reservoir utilisee pour l'evaluation des hydrocarbures en place

Enhanced Resolution Processing of Compensated Neutron Logs

Abstract: Compensated neutron logging (CNL) uses a two-detector system that was developed to reduce borehole effects. The ratio of counting rates from the detectors provides the basic tool response from which a porosity index is obtained. Each detector in this system has a different vertical resolution because of its spacing. A new method of processing the counting rates has been developed to enhance the vertical resolution capabilities of the neutron porosity index by exploiting the better vertical resolution of the near detector. Because no additional or new measurements are required, data from older wells can easily be re-evaluated. Results from the new method have been compared with microspherically focused logs (MicroSFL) and electromagnetic propagation logs (EPT). They show repeatable thin-bed resolution on the order of 1 ft (0.3 m) for data sampled at 6-in. (15-cm) intervals; the typical vertical resolution from ratio processing is approximately 2 ft (0.6 m). The statistical precision of the high-resolution processing is superior to that of the standard ratio method. An additional parameter, obtained with the new processing method, provides information about borehole effects. This parameters can be used for qualitative indications of gas when invasion is not deep and environmental effects are not large. Themore » new method has been applied successfully in carbonate and laminated sand formations. Studies show that thin beds can be detected in high-porosity formations where normal processing has significant statistical variations resulting from reduced counting rates.« less

New Unified Method of Integrating Core Capillary Pressure Data With Well Logs

Abstract: A new unified method is presented for relating porosity, water saturation, capillary pressure (or height in the reservoir), and permeability through multilinear regression with core and log data. This method, an extension of Heseldin, is well-suited to computer processing. In addition to general use in reservoir description, this new method is also shown to be useful in several specific applications.

Behavior of Wells Completed in Multiple Producing Zones

Abstract: Tour d'horizon des methodes d'essai sur reservoir stratifie, notamment des reservoirs dans lesquels le transfert de fluide ne se produit que dans le puits et des reservoirs a ecoulement croise. Les procedures permettant d'obtenir les proprietes caracteristiques des couches (effet de peau, capacite d'ecoulement et pression moyenne) sont detaillees. Les consequences de la stimulation selective des differentes couches sont examinees

Performance Predictions for Gas-Condensate Reservoirs

Abstract: This paper considers some reservoir engineering aspects of the performance of gas-condensate reservoirs that are important for commercial exploitation; calculation of producible fluids, material-balance computations, well deliverability predictions, estimates of the size of the two-phase zone, and saturation changes during the buildup period. Two production modes (constant pressure and constant rate) are considered. Both transient and boundary-dominated periods are examined. Extrapolation of results derived from data above the dewpoint pressure to conditions below the dewpoint pressure is discussed.

Application of the Horner method for a well produced at a constant bottomhole pressure

Abstract: This paper presents an analytical solution to describe the pressure buildup for wells produced at constant bottomhole pressures (BHP's) in an infinite-acting reservoir. The buildup equation is derived on the basis of an initial condition approximating the pressure profile in the wellbore and in the reservoir at the time of shut-in.

Effects of the Quadratic Gradient Term in Steady-State and Semisteady-State Solutions for Reservoir Pressure

Abstract: Constant-rate analytical solutions of the one-phase radial-flow equation in two dimensions, including effects of the quadratic gradient terms, are derived for an oil reservoir with constant diffusivity and compressibility. In this paper the combinations of compressibility contributions to the various terms are analyzed. It is shown that the standard condition allowing the quadratic gradient term to be neglected (crho<<1) is incorrect; pressure, rho, should be replaced by a function of the production rate and other reservoir parameters except absolute pressure. The effect of the quadratic gradient term, for which quantitative expressions are given for steady-state and semisteady-state flow, may amount to several percent of the pressure drawdown at the wellbore.

A Correlation for Interference Testing With Wellbore-Storage and Skin Effects

Abstract: In this paper a new correlation and set of pressure-response and pressure-derivative type curves are presented for interference test analysis. The influence of wellbore storage and skin effects at active and observation wells on interference pressure response is examined. In the case of wellbore storage and skin in one well, the correlating parameter (C/sub D/e/sup 2s/)C/sub D//r/sub D//sup 2/ is used to combine wellbore storage and skin variables in a way that makes it possible to display, on a single type curve, all interference test data that would ordinarily require a number of type curves for various combinations of C/sub D//r/sup 2//sub D/ and C/sub D/e/sup 2s/. The product of the correlating parameter is used when wellbore storage and skin effects are active at both wells. The difference in the shapes of the single-well storage type curves compared to the two-well storage case at early times can be used as a diagnostic features in interference testing.

Application of a Pseudotime Function To Permit Better Decline-Curve Analysis

Abstract: This paper discusses a normalized pseudotime accounting for the effects of system mobility and compressibility used to linearize the post-transient, semilog rate/time plot for a constant-bottomhole-flowing-pressure (BHFP) case. The improved matching technique permits better characterization of reservoir rock parameters, oil-in-place estimation, and prediction of future performance.