Thin Oil Rim Development in the Amherstia / Immortelle Fields, Offshore Trinidad

Abstract

Effective exploitation of the thin oil rim in the Amherstia / Immortelle 22 sand is particularly challenging because of the size of the overlaying gas cap, 2TCF, and the thickness of the oil rim varying between 31 to 46 feet gross pay interval. Since oil wells in the thin oil rim of the 22 sand did not justify well cost, the challenge, then, was to make the oil wells more attractive to facilitate early depletion of the oil rim. Reservoir simulation work was undertaken and a joint team was formed to evaluate the reservoir performance and determine the best strategy for depletion. This team consisted of subsurface professionals from the Amherstia and Immortelle fields, and the bp Houston technology group. The performance of the two wells in the reservoir was history-matched and then the model was used to develop an improved strategy. The reservoir simulation grid used was aligned with the major fault patterns and nested local grid refinement was utilized to better understand depletion of the oil rim. Sensitivity analysis conducted demonstrated that locating longer horizontal oil wells with larger size tubing in the upper third of the oil rim proved to be the best strategy for depletion of the oil rim. The strategy provided higher production rates with earlier accessibility to gas reserves while outrunning the aquifer. Introduction Located 30 to 40 miles off the south - east cost of Trinidad in 235 feet of water (Figure 1), the Immortelle/ Amherstia field complex consists of a number of stacked reservoirs, the largest of which is the 22 sand. This field complex consists of two major NE plunging anticlines; the Immortelle platform lies on the northern-most anticline while the Amherstia platform is perched on the southern-most anticline. A number of faults intersect these two anticlines. The largest of these is the A4 fault. The reservoir consists of a coarsening upward clastic sequence interpreted to be a shore face or delta front environment. Some of the deeper reservoirs, inclusive of the 22 sand, are continuous across the Amherstia / Immortelle field complex.

Flow Rate Effect on Retrograde Condensate Dropout in Trinidad Gas Reservoirs

“…This is to keep the reservoir pressure high and prevent oil moving into the gas zone and being trapped as residual oil. If the oil is only a thin rim beneath a large gas volume, as is the case for some reservoirs off the east coast of Trinidad (Bayley-Haynes and Shen, 2003), the regulating authorities often insist that the oil is produced before gas production. This can delay the gas sales development and have a negative influence on the economics of the hydrocarbon process because the oil income is small.…”

Study on the Reasonable Development Method of Gas Cap Reservoir

“…Ezzam [6] pointed that both gas production and oil production would lead to changes of oil-gas interface and mutual displacement between oil ring and gas-cap. According to Nandalal [7] and Bayley [8], gas channeling into oil ring has little influence on ultimate recovery of natural gas, but oil reduction is inevitable and under some serious circumstance, only gas is produced without any oil in oil production wells. If crude oil enters gas cap, large amount of oil will be wasted and oil recovery will be influenced.…”