The Effect of Oriented Perforations as a Sand Control Method: A Field Case Study from the Varg Field, North Sea

Abstract

Abstract The Varg field located in the central North Sea started production during the fall of 1998. It is located in a complex compartmentalized structure with different pressure regimes. The reservoir quality is generally good exhibiting partly heterogeneous sands containing weak and soft layers and partly homogeneous high-permeable and weak sand bodies. The field stresses are complex due to a number of faults, and indications of stress field rotations in wells in the proximity of faults have been observed. Precise mapping of the stress field has not been possible. Well testing data showed evidence of sand production in one appraisal well, and sand production indications were seen in another well. A sand production risk analysis was conducted prior to completion of the wells, and the risk of sand production was judged substantial. Based on perforation stability computations, selective and oriented perforations were suggested as a method to minimize the sand production risk. A majority of the producers were completed using oriented perforations at 0–180 deg. phasing. Since then no serious sand production events have been reported to date even though reservoir depletion of more than 100 bar (1450 psi) has been experienced in most reservoir compartments. The paper describes the field history with respect to production rates, productivity and sand production, as well as the analysis methodology and input data acquisition. The potential of oriented perforations as a sand control method is discussed on the basis of the field case comparing theoretical computations with field data. Introduction Sand production is one of the most recognized productivity reducing factors in weak sandstone formations. Thus different sand control technologies have evolved over the years with the aim of reducing or eliminating sand production problems. However, active sand control such as gravel packing has often resulted in reduced productivity and additional cost and complexity of operation [1. 2. 3.]. As a response to this, Sand Management has been proposed as an alternative path, for which classical sand control means may play a part, but by no means dominate the picture [4.]. Sand Management involves a thorough understanding of the entire sand "life cycle" from detachment and mobilization at sand face, transportation into wellbore, settling in wellbore or transportation to surface, equipment sand erosion risk, sand monitoring techniques, sand separation, and finally produced sand deposition and eventual cleaning or re-injection. As part of such an approach, the use of oriented perforations represents a design option, for which the understanding of the basic physics of perforation cavity failure and sand production was taken into practice to reduce the formation failure risk. The idea is to shoot the perforations in the most preferable direction with respect to the near-wellbore stress field. The near wellbore stresses are functions of the in situ (reservoir) stress field, which may be computed through an analytical or numerical rock mechanics model describing the elasto-plastic behavior of the formation rock as function of stress. In order to obtain maximum stability, the perforation ought to be oriented such that the stress anisotropy in a plane normal to the cavity axis is minimized. One of the first reports of such an approach in the field was reported by [6.]. The attempt was based on simulations of cavity stability using a Finite Element Model [7. 8. 9.]. The model was later verified and calibrated through laboratory studies [10. 11.]. In particular, the effect of oriented perforations was experimentally verified in the laboratory [12.]. With this background, the use of oriented perforations in sand production management has been adopted in many completions in the North Sea. Vertical perforations are often shot at 180 deg. phasing in horizontal wells in tectonically relaxed basins (i.e. the vertical stress is the major principal stress). With today's technology an accuracy of perforation orientation of +/- 30 deg. or better is realistic.