Coupled Hydrologic, Thermal and Geomechanical Analysis of Well Bore Stability in Hydrate-Bearing Sediments
Authors: Rutqvist Jonny , Moridis George J. 
Published: October 2008

Abstract
In this study we investigated coupled multiphase flow, thermal, thermodynamic and geomechanical behavior of oceanic Hydrate Bearing Sediments (HBS), during depressurization-induced gas production in general, and potential wellbore instability and casing deformation in particular. We investigate the geomechanical changes and wellbore stability for two alternative cases of production using horizontal well in a Class 3 deposit and vertical well in a Class 2 deposit. We compared the geomechanical responses and the potential adverse geomechanical effects for the two different cases. Our analysis shows that geomechanical responses during depressurization-induced gas production from oceanic hydrate deposits is driven by the reservoir-wide pressure decline, DP, which is in turn is controlled by the induced pressure decline near the wellbore. Because any change quickly propagates wihin the entire reservoir, the reservoir wide geomechanical response can occur within a few days of production induced pressure decline. Our study shows that there is a major difference in the geomechanical performance around horizontal and vertical wells. In the case of production from horizontal wells, the anisotropic stress induced by the general reservoir depressurization can cause shear failure near the wellbore adjacent to the perforation. For production from a vertical well on the other hand, the formation will be unloaded uniformly in a plane normal to the axis of the wellbore. Therefore, the load on the wellbore casing will decrease and failure of the formation around the perforation is prevented. In the case of a horizontal well, the main concern is increased compression (load) againt the upper part of the well bore casing caused by the compacting reservoir. This compressive load first caused local shear failure (yielding) in the formation leading to loss of bonding between grains, which may lead to production of solid sediment particles and formation of cavities around the perforation. Our analysis shows that for reasonable strength properties of an oceanic HBS, there is a very high potential for such localized shear failure. In the case of a vertical well, the main concern is the vertical settlement of the formation, which may be substantial, especially in the vicinity of the well where pressure is the lowest. Finally, our analysis shows that the failure of the formation during depressurization-induced gas production is likely to occur at relatively high effective stress. Therefore, investigation of the strength behavior of HBS should be conducted at such appropriate confining stress range, including the possibility of pore-collapse.

Introduction

Background.
Methane hydrates occur naturally offshore in shallow depths below the ocean floor and onshore beneath the permafrost. Hydrates contain enormous quantities of methane gas, which if economically producible will have significant implications for U.S. energy security. Interests in gas hydrates have increased in recent years, with governments as well as several oil and gas producing companies initiating projects for drilling and testing of Hydrate-Bearing Sediments (HBS). Several production methods, including depressurization, thermal stimulation, and inhibitor injection, are being considered for extraction of gas from hydrate-bearing formations. However, the geomechanical response of HBS in general, and potential wellbore instability and casing deformation in particular, are serious concerns that need to be addressed and understood before gas production from hydrate deposits can be developed in ernest. Deposits that are suitable for targest for production oftern involves unconsolidated sediments that are usually characterized by limited shear strength. The dissociation of the solid hydrates (a strong cementing agent) during gas production can induce detoriation of the structural stability of the HBS, which is further exacerbated by the evolution of expanding gas zone, progressive transfer of loads from the hydrate to the sediments, and subsidence. The problem is at its highest intenstity in the vicinity of the well-bore where the largest changes are concentrated, and is further complicated by production-induced changes in the reservoir pressure and temperature. These can significantly alter the local stress and strain fields, with direct concequence on the wellbore stability, the flow and fluid properties of the system, and, consequently on continuing production.