M N Gray

Abstract: Oil and gas wells can develop gas leaks along the casing years after production has ceased and the well has been plugged and abandoned (P&A). Explanatory mechanisms include channelling, poor cake removal, shrinkage, and high cement permeability. The reason is probably cement shrinkage that leads to circumferential fractures that are propagated upward by the slow accumulation of gas under pressure behind the casing. Assuming this hypothesis is robust, it must lead to better practice and better cement formulations Introduction, Environmental Issues This discussion is necessarily superficial, given the complexity of the issue and attendant practical factors such as workability, density, set retardation, mud cake removal, entrainment of formation gas, shale sloughing, pumping rate, mix consistency, and so on. A conceptual model will be developed in this article to explain slow gas migration behind casing, but we deliberately leave aside for now the complex operational issues associated with cement placement and behavior. In 1997, there were ~35,000 inactive wells in Alberta alone, tens of thousands of abandoned and orphan wells, plus tens of thousands of active wells. Wells are cased for environmental security and zonal isolation. In the Canadian heavy oil belt, it is common to use a single production casing string to surface (Figure 1); for deeper wells, additional casing strings may be necessary, and surface casing to isolate shallow unconsolidated sediments is required. As we will see, surface casings have little effect on gas migration, though they undoubtedly give more security against blowouts and protect shallow sediments from mud filtrate and pressurization. To form hydraulic seals for conservation and to isolate deep strata from the surface to protect the atmosphere and shallow groundwater sources, casings are cemented using water-cement slurries. These are pumped down the casing, displacing drilling fluids from the casing-rock annulus, leaving a sheath of cement to set and harden (Figure 1). Casing and rock are prepared by careful conditioning using centralizers, mudcake scrapers, and so on. During placement, casing is rotated and moved to increase the sealing effectiveness of the cement grout. Recent techniques to enhance casing-rockcement sealing may include vibrating the casing, partial cementation and annular filling using a small diameter tube. Additives may be incorporated to alter properties, but Portland Class G (API rating) oil well cement forms the base of almost all oil well cements. Generally, slurries are placed at densities about 2.0 Mg/m, but at such low densities will shrink and will be influenced by the elevated pressures (10-70 MPa) and temperatures (35 to >140oC) encountered at depth. The consequences of cement shrinkage are non-trivial: in North America, there are literally tens of thousands of abandoned, inactive, or active oil and gas wells, including gas storage wells, that currently leak gas to surface. Much of this enters the atmosphere directly, contributing slightly to greenhouse effects. Some of the gas enters shallow aquifers, where traces of sulfurous compounds can render the water nonpotable, or where the methane itself can generate unpleasant effects such as gas locking of household wells, or gas entering household systems to come out when taps are turned on. Methane from leaking wells is widely known in aquifers in Peace River and Lloydminster areas (Alberta), where there are anecdotes of the gas in kitchen tap water being ignited. Because of the nature of the mechanism, the problem is unlikely to attenuate, and the concentration of the gases in the shallow aquifers will increase with time. This implies that current standards for oilwell cementing and P&A are either not well founded, or the criteria are based on a flawed view of the mechanism. This is not a condemnation of industry: all companies seek to comply with standards. Nevertheless, we believe that the AEUB Interim Directive 9903 is flawed with respect to gas leakage around casings. To rectify this, the mechanisms must be identified correctly. Practise can then be based on correct physical mechanisms, giving a better chance of success (though we do not believe SPE 64733 Why Oilwells Leak: Cement Behavior and Long-Term Consequences Maurice B. Dusseault, SPE, Porous Media Research Institute, University of Waterloo, Waterloo, Ontario; Malcolm N. Gray, Atomic Energy of Canada Limited, Mississauga, Ontario; and Pawel A. Nawrocki, CANMET, Sudbury, Ontario 2 DUSSEAULT, GRAY AND NAWROCKI SPE 64733 that the problem can be totally eliminated because of the vagaries of nature and human factors, despite our best efforts). There is also need for better quality oil-well cement formulations that can resist thermal shocking. For example, leakage of fluids along thermal wells in cyclic steam operations in Alberta has proven a challenging problem for Imperial Oil. If poor quality or poorly constituted cement is used, high injection pressures, thermal shocking, plus non-condensible gas evolution lead to leakage behind the casing that could break to surface under exceptional conditions. Finally, in production management for conservation purposes, zonal isolation is multiple-zone wells. There are initiatives to identify old leaking wells and undertake mitigating action in Alberta and Saskatchewan, the “orphan well” program of the AEUB, initiatives by the Petroleum Technology Alliance Centre in Calgary, and so on. This article is to try and clarify the mechanisms involved.

TL;DR: In this paper, a study of the maximum attainable densities of candidate Na bentonite/sand and illite/sand buffers is described, and the addition of sand significantly increases the achievable compacted density.

TL;DR: Two large-scale sealing experiments were conducted at Atomic Energy of Canada Limited's underground research laboratory at Lac du Bonnet, Manitoba as discussed by the authors, where the rate of water uptake in densely compacted san...

TL;DR: In this article, the authors describe low-gradient tests on compacted illite clays for hydraulic conductivity and show that the clays normally function at hydraulic gradients much lower than are commonly used in laboratory tests for hydraulicconductivity.

TL;DR: Triaxial and one-dimensional swelling tests have been performed at pressures up to 3'MPa on a compacted sand-bentonite mixture with potential use for containing nuclear and other toxic wastes as discussed by the authors.