Production Strategy for Thin-Oil Columns in Saturated Reservoirs

Abstract

Abstract Maximizing oil recovery in thin and ultra-thin (< 30 ft) oil columns is a challenge because of coning or cresting of unwanted fluids, regardless of well orientation. Significant oil is left behind above the well completion even for horizontal wells when bottom- or edge-water invasion occurs. Two depletion strategies may be enacted to improve recovery of the remaining oil. In the first option, a conventional horizontal is completed below the gas/oil contact (GOC). Once the well waters out, the well is recompleted in the gas zone. Completion occurs either at the crest for a small gas-cap reservoir or at the GOC, inducing reverse cone, for reservoirs with thick-gas columns. Alternatively, one can skip the initial oil completion. Gravity-stable flooding is required to maximize reserves. Extensive flow simulations in multiple, history-matched models have shown that the proposed strategy improves recovery significantly. Two field examples are presented to demonstrate the usefulness of the proposed method. Using multivariate regression, simple correlations were developed for quick screening of the proposed approach. Experimental design formed the backbone of a parametric study involving various reservoir, fluid, and process variables. We tested and validated the correlations with independent sets of experimental and published field data. Introduction In the early days of horizontal drilling, successful reserves exploitation of thin-oil columns were reported in North Sea,1 Australia,2 and Gulf of Mexico,3 among others. Nonetheless, economic exploitation of ultra-thin-oil columns (< 30 ft) is relatively new. For instance, Vo and his co-workers4–6 have shown that successful depletion can be effected with densely spaced horizontal wells. Dense well spacing, such as 30 acres in the Attaka field,5 certainly calls for inexpensive drilling. However, economics may dictate pursuing other measures when high-density drilling becomes infeasible. Placement of horizontal wells in a thin-oil column (< 40 ft) is a challenge and depends on relative drive indices of the gas cap and the aquifer. Typically, the gas-cap expands easily as depletion occurs in the system. However, depending on the strength and connectivity of the aquifer, a time-delayed response occurs. The gas/oil contact recedes with water influx. Ultimately, cresting of water causes the well waters out. Even when good production practices are adhered to, a significant oil column is left behind at abandonment. In other words, the standoff between the GOC and horizontal well may leave upwards of 10+ ft of oil column. There are two ways to capture this oil. Either we allow the aquifer or injected water to drive this oil into the gas cap, and finally into a crestal well, or else place the horizontal well near the GOC, just above or below it, to minimize the loss of oil. The first option is feasible when the gas-cap is relatively small. In fact, Behrenbruch and Mason7 proposed the notion of gas-cap blowdown as a recovery mechanism for reservoirs with small gas-to-oil-column-thickness ratio, less than 20%, in strong water-drive reservoirs. However, our work shows that systems with much higher in-place gas/oil ratio (OGIP/OOIP) with moderate water-drive systems can lend themselves to blowdown, provided good vertical reservoir continuity exists. To avoid displacing oil into large gas-caps and consequent ‘smearing,’ we advocate the notion of reverse coning of oil by placing a horizontal well just above the GOC. As expected, very high-initial GOR production will be experienced before significant oil rate takes effect. Where gas monetization is not an issue, this approach is recommended to maximize oil recovery. However, to mitigate excessive early gas production, the well can be placed just below the GOC. Irwin and Batycky8 showed that smearing of oil in a thick-gas column did not occur. In carbonate reef reservoirs, they found that even an 18-ft oil column could be successfully displaced by the bottom water into a 500-ft gas column, without experiencing the anticipated loss of oil. The main objective of this study is to discuss strategies for reserves exploitation in thin-oil columns, regardless of the size of the associated gas-cap. Production Strategy Conventional scheme involves the use of horizontal or vertical wells for reservoir development under natural depletion. The ideal production scenario involves oil withdrawal with minimal depletion from the gas-cap to minimize energy loss.