SUMMARYDeformation mechanisms at the pore scale are responsible for producing large strains in porous rocks. They include cataclastic flow, dislocation creep, dynamic recrystallization, diffusive mass transfer, and grain boundary sliding, among others. In this paper, we focus on two dominant pore-scale mechanisms resulting from purely mechanical, isothermal loading: crystal plasticity and microfracturing. We examine the contributions of each mechanism to the overall behavior at a scale larger than the grains but smaller than the specimen, which is commonly referred to as the mesoscale. Crystal plasticity is assumed to occur as dislocations along the many crystallographic slip planes, whereas microfracturing entails slip and frictional sliding on microcracks. It is observed that under combined shear and tensile loading, microfracturing generates a softer response compared with crystal plasticity alone, which is attributed to slip weakening where the shear stress drops to a residual level determined by the frictional strength. For compressive loading, however, microfracturing produces a stiffer response than crystal plasticity because of the presence of frictional resistance on the slip surface. Behaviors under tensile, compressive, and shear loading invariably show that porosity plays a critical role in the initiation of the deformation mechanisms. Both crystal plasticity and microfracturing are observed to initiate at the peripheries of the pores, consistent with results of experimental studies.

SUMMARYWe develop a computational framework that captures the microfracture processes leading to shear band bifurcation in porous crystalline rocks. The framework consists of computational homogenization on a representative elementary volume that upscales the pore-scale microfracture processes to the continuum scale. The assumed enhanced strain finite element approach is used to capture the discontinuous displacement field generated by the microfractures. Homogenization at the continuum scale results in incrementally nonlinear material response, in which the overall constitutive tangent tensor varies with the stress state and with the loading direction. Continuum bifurcation detects the formation of a shear band on the representative elementary volume level; multi-dimensional strain probes, necessitated by the incremental nonlinearity of the overall constitutive response, determine the most critical orientation of shear band bifurcation. Numerical simulations focus on microfracturing at the pore scale with either predominant interface separation or predominant interface contact modes. Results suggest a non-associative overall plastic flow and shear band bifurcation that depends on the microfracture length and the characteristic sliding distance related to slip weakening.

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Open hole wireline logging, due to its lower overpull capacity relative to logging-while-drilling, is subject to increased stuck tool risk and can lead to costly fishing operations, lost-in-hole charges, and the need to sidetrack the borehole. The risk increases in higher overbalance environments typical for depleted reservoirs but there are other controls on sticking risk that can be managed. This paper describes the measures taken in an open hole wireline logging operation in a Deepwater Gulf of Mexico well under close to 7000 psi overbalance in a high temperature-high pressure (HTHP) slim-hole environment using water-based reservoir drilling fluid (RDF) to achieve a successfully efficient logging operation without incurring lost time attributed to conveyance related issues. A wireline program was designed that paid close attention to the RDF properties to ensure excellent fluid loss performance and a thin filter cake, a recipe to keep differential sticking in check. The target reservoir was an aeolian sandstone with a porosity of 19% and permeability in the tens of millidarcy range. Additional steps taken to mitigate sticking risks included good borehole cleaning practices, sticking assessment during drilling, use of high-pull capacity cable, minimization of tool contact area with the borehole through reduced tool string length and use of rollers and standoffs, minimization of stationary time within depleted reservoir, tension monitoring during wireline operation, and inclusion of a jar in the tool string to increase the chance of pulling free when stuck. A high temperature and pressure rated sampling tool was run in a 7″ borehole with a 25° open-hole deviation to acquire the pressure profile in the reservoir. Correlation was performed inside the casing such that the only stationary time the tool string experienced was during the pressure sampling operation. The sampling itself was limited to no more than ten minutes. As a result of the mitigation measures implemented, a total of five good pressure points were obtained in approximately 150 ft of reservoir interval. Negligible tension drop-off was observed when moving down from one pressure station to another and no overpull was observed when picking up the tools. The tool string was successfully retrieved at the surface. From the authors’ knowledge, this is the first time logging under such high overbalance has been performed with a water-based RDF. The fluid mentioned in this paper has been engineered to overcome the typical shortcomings of water-based RDF when it comes to differential sticking. Learnings from stuck tool experiences during previous wireline logging attempts in the same field will be described, providing the rationale for some of the mitigation steps pursued in this paper. The success of differential sticking mitigation during this logging program opens the door to possibilities for performing future open hole wireline logging under extreme conditions.