This paper quantifies the relationship between rock compaction and well flow by including an anisotropic, stressdependent permeability tensor. When producing oil from a weak reservoir, rock compaction induced by pressure drawdown may occur, often with adverse consequences. For weak reservoirs such as unconsolidated sands, rock compaction often causes permeability reduction that may significantly influence the well productivity to the extent that production becomes uneconomic. A 3D finite element model was developed in order to simulate fluid production through a well in a deforming reservoir. Constitutive models for weak reservoir rock and deformation-dependent permeability tensor are also supplemented in the finite element model. The developed model was used to evaluate the influence of rock compaction on well productivity for compaction-sensitive formations. Results show that the model provides an effective tool to identify possible mechanisms associated with rock compaction that cause permeability reduction. The improved understanding of the permeability reduction mechanisms achieved by the model provides a guide to mitigate well productivity impairment resulting from the compaction of these deformable reservoirs. Numerical simulations for different reservoir characteristics, operating conditions, and well configurations were performed in order to establish quantitative relationships between well productivity declines and key field operational control variables. Modeling results for representative cases and the description and formulation of the finite element model are also presented.
1.INTRODUCTION
Fluid production of a hydrocarbon reservoir results in decreasing fluid pressure and increasing effective overburden load on reservoir rock. The increase in effective overburden load will in turn compact the reservoir rock and change the stress state in the reservoir. Significant permeability reduction associated with rock compaction around the wellbore region is a well-known phenomenon in oil and gas production in weak reservoirs. As a consequence, rock compaction has adverse effects on well productivity.
To quantify well productivity reduction caused by rock compaction, a coupled analysis is required because the physical process involves both geomechanics and fluid flow. Also, in a geologically and geometrically complex setting, it is very difficult, if not possible, to analyze the coupled problem analytically or semi-analytically. On the other hand, advances in numerical computing technology have made the numerical modeling of the coupled geomechanics and fluid flow problem rigorous, robust and efficient in a general fashion.
Thus, the objective of this paper is to develop a general 3D finite element model that couples geomechanics and fluid flow, to quantify well productivity reduction induced by rock compaction for practical production operations.
In the following sections, first, we present the details of the numerical model that include coupled field equations, initial and boundary conditions, constitutive relations, and numerical procedures. Second, the model was verified for its accuracy and its ability of retaining primary physics by test problems. Third, to demonstrate the capabilities of the model for practical field applications, we conducted modeling studies to examine and discuss how well productivity loss is influenced by operating variables such as production rate, well configurations, and reservoir characteristics including permeability anisotropy and layered heterogeneity in a compacting reservoir. Lastly, conclusions are drawn from this work.
