ABSTRACT:

The success of infill drilling operations in a deep, high-pressure, high-temperature (HPHT) field in the North Sea depends in part on our ability to predict the deformation and stress changes in the reservoir and overburden formations, resulting from hydrocarbon recovery activities and induced reservoir depletion. These changes can lead to several challenges, including, among others, the closure of the mud weight window due to a substantial reduction in the minimum total principal stress or fracture pressure, the loss of well integrity due to a significant liner deformation caused by the re-activation of pre-existing geological faults or weak lithology interfaces, and potential sand production problems due to pore collapse caused by critical plastic deformations. This paper focuses on the construction, calibration and application of a geomechanical model as a predictive tool to assist the understanding of a mechanically complex HPHT field. The geomechanical model proved to be a useful tool to explain field measurements and observations, such as the reduced fracture gradient in the overburden and well failures. In addition, it may assist future field development. Examples of operational support include mud-weight prediction for stable wells, location of relatively safe areas to drill (e.g. with low-slip risk), and advice on drawdown and completion design to prevent or mitigate sand production.

1. INTRODUCTION

Production-induced depletion of high-pressure, high temperature (HPHT) fields can lead to significant deformation and stress changes both inside and outside the reservoir [1-6]. Outside the reservoir, the decrease in reservoir thickness leads to straining of the rock matrix and therefore the in-situ stress state will change there too. These changes may result in potential negative consequences to the economic performance of the field, among others, including seabed subsidence and platform sinking, porosity and permeability loss, pore collapse and sand production, fault slip and bedding-parallel slip, casing deformation and well failure. Some of these have been extensively addressed and reported in the literature over the past decades [7-11]. The HPHT field studied here also faces potential problems. This is a complex faulted gas condensate field located in the UK Central Graben of the North Sea, where the large depletion of the reservoir of about 60 MPa, in combination with the high reservoir uniaxialstrain compressibility of about 1.5 x 10-4 /MPa, have led to reservoir compaction and associated overburden stretching. The presence of faults, in turn, has resulted in the loss of wells, presumably by fault-slip-induced casing breach. Since considerable volumes of gas still remain in place, infill drilling is required to be able to access these volumes, either by sidetracking existing damaged wells or by drilling infill wells. This paper differs from previous work in its focus on the change in the in-situ stress state that may influence the stability of new wellbores and the integrity of new wells (e.g., deep liner deformation that results in loss of communication with the reservoir). This poses challenges for infill drilling operations. The biggest challenge is to drill a stable wellbore to prevent collapse, gas kicks, and mud losses.

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