Maurice B. Dusseault

Casing Shear: Causes, Cases, Cures

Casing impairment leads to loss of pressure integrity, pinching of production tubing, or an inability to lower workover tools. Usually, impairment arises through shear owing to displacement of the rock strata along bedding planes or along more steeply inclined fault planes. These displacements are shear failures. They are triggered by stress concentrations generated by volume changes resulting from production or injection activity. Volume changes may arise from pressure changes, temperature changes, or solids movement (solids injection or production). Dominant casing-deformation mechanisms are localized horizontal shear at weak lithology interfaces within the overburden; localized horizontal shear at the top of production and injection intervals; and casing buckling within the producing interval, primarily located near perforations. Mitigating casing damage usually means reducing the amount of shear slip or finding a method of allowing slip or distortion to occur without immediately affecting the casing. Strengthening the casing-cement system seldom will eliminate shear, although in some circumstances it may retard it. Proper well location or inclination, underreaming, special completions approaches, reservoir management, and other methods exist to reduce the frequency or rate of casing shear.

Drilling in extreme environments : penetration and sampling on Earth and other planets

1 Introduction 2 Principles of Drilling and Excavation 3 Ground Drilling and Excavation 4 Ice Drilling and Coring 5 Underwater Drilling 6 Extraterrestrial Drilling and Excavation 7 Planetary Sample Acquisition, Handling and Processing 8 Instruments for In-Situ Sample Analysis 9 Contamination and Planetary Protection 10 Conclusions

A coupled conductive–convective thermo-poroelastic solution and implications for wellbore stability

Abstract Steam injection is widely used in heavy oil reservoirs to enhance oil recovery; elevated temperatures increase fluid mobility in several ways, but can also generate damage through shearing, crushing of weak grains, and casing impairment by shear, collapse, or buckling. Disposal of cold produced water by injection can generate thermally induced extensional fracturing, increasing the effective wellbore radius. Drilling with long open-hole sections can lead to rock temperature changes as large as 30–40 °C at the casing shoe through mud heating at depth and upward mud circulation, dramatically impacting wellbore stability. Clearly, thermal stress analysis of open and cased boreholes is of primary interest for drilling and completion planning, as bottom-hole temperature changes can have as large an impact as bottom-hole pressure changes. Local wellbore stresses are the sum of far-field, pore pressure and thermally induced stresses; they may be highly inhomogeneous because of different rock properties and heat transport processes. These stresses, combined with thermal weakening and pore pressure changes, may lead to phenomena such as formation damage, sand production, shale shrinkage, and various modes of instability (shearing, spalling, fracturing, etc.). Previous studies of thermally induced stresses were primarily based on assumptions of low permeability and heat conduction only; this is inadequate when high-permeability formations are encountered. To analyze induced stresses and formation damage, a geomechanics model that is fully coupled to diffusive transport processes is employed. By assuming a constant wellbore pressure and temperature boundary condition, a closed-form solution including heat conduction and convection is obtained for the stresses near a cylindrical wellbore. The stability of an open-hole subject to non-isothermal, non-hydrostatic in situ loading and various conditions is then investigated. Our studies indicate that maximum tangential stresses are found on the wellbore wall during production, but can be displaced into the formation if a cooler wellbore fluid is used. This is a more stable condition because of higher confining stresses deep within the borehole wall.

Why Oilwells Leak: Cement Behavior and Long-Term Consequences

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.

Description of fluid flow around a wellbore with stress-dependent porosity and permeability

Abstract Stress-dependent porosity and permeability effects have been widely studied at the laboratory scale, as they can significantly affect reserve estimates, well production rate and profitability. Based on current experimental data and theories, a general analytical method of calculating stress-dependent porosity and permeability is developed and applied to a wellbore producing oil from unconsolidated or weakly consolidated sand, with the aid of a coupled geomechanical model by which stress distributions around the wellbore can be specified. For clean weak sand, nonlinear elastic theory is appropriate for calculations of stress-dependent rock properties such as compressibility, porosity and permeability. When evaluated in terms of pore pressure variations, the stress-dependent aspect of porosity and permeability may be negligible as far as stress analysis concerned. With input of different stress-compressibility relationships, the model can be used to help screen those reservoirs for which the effect of stress on permeability should be considered during geomechanical analysis (sand production prediction, reservoir stress arching and shear, plasticity onset, etc.). Also, it can be used for analyzing formation compaction that results from the decrease of stress-dependent porosity. The model limitations have been discussed and it is believed that a microscopic approach based on particulate mechanics may be valuable for future research. Different boundary conditions commonly used in current geomechanics models have been compared and discussed in the development of the poro-inelastic geomechanics model, and boundary restraint is demonstrated to be a critical factor to stress solutions.

Microseismic Monitoring Developments in Hydraulic Fracture Stimulation

The last decade has seen a significantly increased interest in microseismic monitoring by the hydrocarbon industry due to the recent surge in unconventional resources such as shale-gas and heavy-oil plays. Both hydraulic fracturing and steam injection create changes in local pore pressures and in situ stresses and thereby brittle failure in intact rock plus additional slip/shearing in naturally fractured rock. Local rock failure or slip yields an acoustic emis‐ sion, which is also known as a microseismic event. The microseismic cloud represents thus a volumetric map of the extent of induced fracture shearing, opening and closing. Microseis‐ mic monitoring can provide pertinent information on in situ reservoir deformation due to fluid stimulation, thus ultimately facilitating reservoir drainage. This paper reviews some of the current key questions and research in microseismicity, ranging from acquisition, proc‐ essing to interpretation.

Behaviour of the clay/bitumen/water sludge system from oil sands extraction plants

Abstract Rational waste management for large mineral extraction operations requires behavioural information and a clear understanding of processes. The behaviour of the clay/water/bitumen/sand systems at Syncrude Canada Ltd. is being studied. The macroscopic behaviour is becoming well understood, particularly with respect to the role of bitumen. The bitumen aids settlement, but hinders consolidation. Clay mineral behaviour in the presence of bitumen is unclear and remains worthy of detailed study, particularly with respect to the nature of the mineral/bitumen interface in the presence of various ions. Data are presented on system rheology, gel strength of sand/sludge mixes, the void ratio to permeability relationships for sludge, and the void ratio/effective stress relationship for sludge. Of considerable interest are the data at extremely low stresses, less than 1.0 kPa, as the great majority of water is expelled below this stress value.

The Effect of Quadratic Gradient Terms on the Borehole Solution in Poroelastic Media

When compressible fluid continuity in a fluid-saturated compressible porous medium under transient conditions is considered, we cannot obtain a standard linear diffusion equation in terms of pressure unless we ignore the quadratic terms in the pressure gradient expression, for example [∂p/∂r]2 for cylindrical plane strain coordinates. They are assumed to be so small that their contribution can be ignored in pressure analysis. Thereby, a nonlinear equation can be avoided. During hydraulic fracturing, rapid drawdown, or slug testing, the pressure difference can reach a high value “instantly.” An extremely steep pressure gradient is generated, and it may not be appropriate to neglect quadratic terms. In this paper, an analytical solution for pore pressure coupling with the deformation in a porous medium is developed by taking the quadratic term into account. By Laplace transformation, we obtain a solution for a nonlinear diffusion equation by setting up a fluid continuity equation according to the mass conservation law rather than from energy principles in terms of volume. Deviations from existing solutions are identified in cases of high pressure gradients, and these deviations are related to the compressibility of the pore and injected fluids. It would seem that the new solution gives a more correct early time response in these cases. To calculate the effective stresses and pore pressure, we need to carefully define hydraulic diflfusivity. We have related the coefficients of Biot and Geertsma to those of mass conservation, which are commonly used in hydrology.

Drilling in mudrocks: rock behavior issues

Abstract Borehole stability analyses use different analytical models to evaluate stresses, pressures, and temperatures around boreholes. Mudrocks do not possess spatio-temporally constant material properties: mechanical properties are time-dependent because of the continuous change in pore pressure, moisture content, and temperature near the borehole. Mudrocks with high specific surface areas also exhibit high sensitivity to changes in physical and chemical properties of their pore fluids. Pressure and concentration diffusion during drilling continuously change the ionic composition of pore fluids and, accordingly, the engineering properties of those fine-grained argillaceous materials. Such fine-grained materials are considered “reactive”; it is shown that a “reactivity coefficient” can be used to assess the physico-chemical sensitivity of mudrocks. Coupling between chemical and mechanical properties may develop in reactive mudrocks, leading to further changes in their mechanical parameters. Experimental data from triaxial and oedometer tests are presented for several materials to illustrate their time-dependent mechano-chemical properties in borehole environments. A discussion of coupled diffusivity processes in mudrocks is included, and the concept of large micro-scale strains that can lead to deterioration of properties is introduced. Finally, we speculate on the degree of complexity required for constitutive modeling of borehole stability analysis of shales in practical situations.

Fluid rate enhancement from massive sand production in heavy-oil reservoirs

Abstract In this paper, we consider sand production instability as the result of wellbore fluid pressure reduction (drawdown) below some critical level. A continuum model for massive continued sand production is developed by coupling fluid flow and granular matrix flow. This model shows the typical significant fluid rate enhancement which accompanies continuous sand production. The model provides a physical mechanism for propagation of a yielding front away from the wellbore as the process of sand production continues. Mass-balance analysis leads to a relation between cumulative sand production and the current yielding front location. Short-term enhancement of fluid production because of simultaneous sand production turns out to be dependent not only on instantaneous sand flux, but also on the current radius of the yielded zone around wellbore, that is, the history of sand production for the well. Long-term enhancement is stipulated mainly by growth of a yielded, highly permeable zone around the wellbore.