For the compacting reservoirs of the North Sea, acid fracturing has shown to be an economical means of stimulating vertical wells. However, when applying the same practice in horizontal wells, the wells often encounter casing deformation problems significantly earlier than vertical wells. A 3D finite element model that simulates fluid production through a well in a deforming reservoir was developed. The model was used to analyze the influence of well stimulation on the rate and morphology of horizontal well casing deformations. Results from the model show that the model provides an effective tool to identify possible mechanisms that cause casing deformations in horizontal wells treated by acid fracturing. The improved understanding of the deformation mechanisms achieved by the model provides a guide in the selection of the most appropriate treatment for a horizontal well in chalk reservoirs. Numerical simulations over a wide range of operating conditions were performed using the model to establish quantitative relationships between casing deformation and key control variables. Modeling results for representative cases and the description and formulation of the finite element model are also presented in the paper.
For hydrocarbon reservoirs with weak formations, high frequency of well casing deformation induced by reservoir compaction was generally observed during oil/gas production operations (e.g., Yudovich et al. 1989, Schwall & Denney, 1994, Fredrich et al. 1998, Nagel, 2000). In general, the casing deformation mechanisms are complicated and inextricably linked with reservoir characteristics, in-situ geology, well configuration and completions, and reservoir production management. Different types of casing deformation mechanisms, that include shear, buckling, bending, traction, and compression, etc., can be triggered by the reservoir compaction process, Thus, for a specific oil/gas field, it would be very difficult to implement a program for mitigating casing deformation without a better understanding of the physical mechanism that caused the problem. Finite element modeling has been employed by many investigators (e.g., Chia & Bradley 1989, Yudovich et al. 1989, Hamilton et al. 1992, Hilbert et al. 1998) as an effective tool to gain insights of casing deformation problems caused by reservoirock compaction. In particular, finite element models that properly simulate the primary physics of a casing deformation mechanism, can provide useful quantitative information and an increased understanding of the physical parameters of the mechanism.
In this paper, a 3-dimensional, finite element model was developed to numerically investigate a horizontal well casing deformation problem induced by well stimulation in a compacting reservoir. Acid fracturing has been a common practice for stimulating vertical wells to improve productivity in the chalk reservoirs at the North Sea. When applying the same practice in horizontal wells, these wells experience casing deformation problems much earlier than in vertical wells. Based on casing deformation data from field measurements, it is postulated that well stimulation by acid fracturing may create cavities around the perforated zone of the treated well. For a horizontal well, the created cavity can result in vertical load shifting along the axis of the horizontal well. The shifted vertical load will induce sharply higher stress increases in casing and speedup the casing collapse process. For a vertical well, the created cavity will cause horizontal load shifting along the vertical direction of the well. During oil/gas production, this effect of horizontal load shifting for the vertical casing may give a slower rate on casing collapse
