Prediction of Wellbore Stability Using 3D Finite Element Model in a Shallow Unconsolidated Heavy-Oil Sand in a Kuwait Field

Abstract

Abstract As a part of field development campaign to produce heavy-oil from a shallow sandstone reservoir, among vertical wells, drilling of horizontal wells was considered as an option. However, due to the weak and unconsolidated nature of the reservoir sand, the stability of horizontal wellbore during drilling was considered a major unknown. The stability of rock around the wellbore during drilling is function of several factors including rock strength, in situ stresses, pore pressure and drilling parameters. Integration of a wide variety of data into a geomechanical analysis is required for wellbore stability predictions. A comprehensive geomechanical study of the unconsolidated sandstone reservoir was conducted by incorporating data from six vertical offset wells in order to constrain the contemporary state of stress and rock strength profiles as a function of depth. The aim of the study was to evaluate the risk of hole stability during drilling, recommend a mud weight for drilling upcoming horizontal wells and to suggest azimuths along which horizontal wellbores will be more stable during drilling and production. To closely simulate the actual stress state and rock deformation around the wellbore during drilling, a 3D Finite Element-based wellbore model was built. The analysis was performed for a range of mud weights to analyze the sensitivity of wellbore stability to mud weight variations. Based on the study, a mud weight was recommended to drill the planned horizontal wells. In addition, the analysis implied that for the observed stress state in the study field, horizontal wells oriented sub-parallel to the minimum horizontal in situ stress would be more stable during drilling and production compared to those oriented sub-parallel to the maximum horizontal in situ stress. Introduction As part of a field development campaign and production strategy for a heavy oil sand in Kuwait, both cold production and steam injection are being considered. While the majority of wells will be drilled vertical, a good number (~30%) of future wells are planned to be horizontal. Due to the nature of envisaged production operations, field development strategy and the characteristics of unconsolidated sand, a geomechanics study was conducted to investigate the risk of wellbore stability during drilling and to suggest a mud weight for successful drilling of horizontal wells to optimize field development campaign. Apart from wellbore stability issues, drilling a stable and less rugos hole improves the quality of openhole logs for accurate petrophysical analysis and formation evaluation, ensure better cement jobs to reduce risk of water channeling, and achieve deeper perforations to access reservoir behind the damaged zone. Moreover, due to porous and permeable nature of the sand, an optimum mud formulation is required to reduce formation damage and limit the invaded zone around the wellbore. The geomechanical analysis was focused on characterization of present-day in situ stresses, rock mechanical properties and rock strength parameters. This objective was achieved by integrating all available information from laboratory tests, drilling events, logs data and downhole measurements. The main objective of the study was to use this information coherently in an integrated geomechanical analysis to recommend safe and optimum mud weight for drilling of upcoming horizontal wells. While the short term focus of study was to suggest stable wellbore directions, the geomechanics understanding and knowledge of the field would be used in subsequent stages of the field development and production optimization.