Borehole stability analysis accounting for anisotropies in drilling to weak bedding planes

Abstract

Borehole instabilities pose significant challenges to drilling and completion operations, particularly in regions with weak bedding planes and pre-existing fractures where formations have strong anisotropies. The bedding planes, rock anisotropy, and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling. This improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions. Laboratory test data of rock compressive strengths are analyzed, and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities. Time-dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time. The slip failure gradient in the weak planes is derived, which can be used to model wellbore sliding/shear failure in the planes of weakness. The mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model.

Introduction

Borehole instability is a major cause of borehole failures and represents a serious challenge in the drilling industry. A lack of accurate wellbore stability analysis brings many problems, such as borehole washouts, breakout, collapse, stuck pipes and drill bits, and losses of boreholes. Wellbore instability also adds to drilling time, increased costs, and sometimes leads to abandoning the well before it reaches its objective. Estimates put the cost of these issues at approximately 10% of total drilling time on average [1]. The relationship of mud weight and wellbore failures (Fig. 1) demonstrates that when the mud pressure is less than the pore pressure, the wellbore has splintering failure or washout. When the mud pressure is less than the shear failure gradient, the borehole has shear failure or breakout/collapse. If the mud weight is higher than the fracture gradient, the drilling-induced hydraulic fractures are generated, causing drilling mud losses or lost circulation. To maintain borehole stability, the applied mud weight should be in an appropriate range. The borehole failures can primarily be classified to the following four categories as illustrated in Fig. 1: (1) wellbore washouts or fluid kicks due to underbalanced drilling, where the mud weight is much less than the pore pressure; (2) wellbore breakouts or shear failures due to a low mud weight; (3) mud losses or lost circulation due to tensile failure (hydraulic fractures) induced by a high mud weight; and (4) rock failures or sliding related to pre-existing fractures.

Different analytical methods and numerical models have been used for borehole stability analyses [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. However, borehole instability is still a main cause of borehole losses in difficult formations and conditions, such as unconsolidated formations, faulted and fractured rocks, weak planes, rubble zones, and salt structures. Therefore, more sophisticated geomechanical modeling is required for accessing the reservoirs under these difficult conditions. For instance, drilling along bedding planes and in depleted reservoirs is very risky [21]. When a well is drilled at shallow angles to thinly bedded shales, it is often highly unstable. Rock failure can occur as a result of rock strength anisotropy caused by weak bedding planes. In these cases, an increased mud weight while drilling is required. However, when the reservoir immediately beneath the bedded shales is depleted, the increased mud weight can lead to lost circulation. Modeling of this geomechanical environment presents many challenges and requires coupling the in-situ stress, pore pressure, mud pressure, and anisotropic effects of rock strengths and stresses. Borehole stability modeling with considerations of pre-existing fractures and planes of weakness in oil and gas wells has been reported (e.g., [17], [21], [22], [23], [24], [25], [26], [27], [28]), but failure mechanism of boreholes in planes of weakness is still not fully understood. This paper first introduces borehole stability analysis in isotropic rocks with emphasis on how to determine the input parameters for the modeling, including in-situ stress and rock strength. Then, the rock strength anisotropy and weak bedding plane impact on borehole stability are studied.