Breathing New Life Into a Thin Oil Column by Horizontal Drilling

Abstract

Abstract The Shuaiba reservoir is one of a stacked system of reservoirs in the Saih Rawl field in Central Oman. Relative to other reservoirs in the field, development of the tight Shuaiba limestone has been unattractive because of rapid water breakthrough in vertical wells completed in the thin oil column. The production performance of a trial horizontal well, drilled in 1991, has been very encouraging. A three-fold increase in oil productivity has been observed, indicating that encroachment of water to horizontal wells is significantly less of a problem than coning to vertical wells. Simulation models have been constructed and used to history match the performance of vertical wells and predict the performance of full field development by horizontal drilling. Optimum well spacing has been identified for various values of oil column height while sensitivities to other identified key variables, such as reservoir quality have been evaluated. The ultimate recovery from the Saih Rawl Shuaiba reservoir can be significantly increased via development by horizontal drilling, at an appreciably lower unit cost than development with vertical wells, thus confirming the potential for a sizeable development project. Introduction The Saih Rawl Shuaiba reservoir was discovered in 1971. The reservoir contains undersaturated light oil in a thin column (less than 25m) underlain by water, trapped in a flat but slightly undulating structure (Figures 1 - 3). Although the reservoir is penetrated by 28 wells, these are mainly to deeper (and in the past economically more attractive) reservoirs. Only 3 wells (which are all vertical) have been completed on the Shuaiba reservoir itself, SR-16, SR-20 and SR-23. Due to bottom water coning in the thin oil column these wells have produced relatively little oil and therefore discouraged further development with vertical wells. Relevant reservoir data are listed in Table 1. With the development of horizontal drilling, it was decided in 1991 to drill and test a horizontal sidetrack of well SR-23. The production performance of this 350 m horizontal sidetrack over the past 18 months, illustrated in Figure 5, shows very promising results compared with the well's previous performance as a vertical producer. The initial net oil rate was 120 m3/d, equivalent to three times the initial net oil productivity of a vertical well. Subsequently, reservoir simulation study has been carried out to support further field development with horizontal wells. This paper describes how the development potential of this reservoir by horizontal drilling was assessed, with only a limited amount of historical production data but with a large number of well penetrations which clearly define the range in reservoir quality over the field. Uncertainty was initially reduced by matching the performance of vertical wells using a single well radial simulation model. This provided the basic data set to construct a horizontal well model which was used to investigate performance for a wide range of sensitivities. Specifically, the model was used to predict performance P. 215^