Horizontal Waterflooding in Shuaiba Carbonate Reservoir of Daleel Field in Oman: From Pilots' Performance To Development Era

Abstract

Daleel hydrocarbon field is located in Block-5 concession area in North Oman. The field produced oil from a Shuaiba carbonate reservoir for more than ten years in depletion mode initially with vertical wells and then horizontal wells. Due to high depletion in the field and the need of redeveloping the field, horizontal waterflooding was selected as the best mechanism to achieve this after scrutinizing several alternatives. To confirm the selection of the horizontal waterflooding, several pilots were implemented in selected areas of the reservoir, with closely monitoring the performance and behavior of existing and new wells. Pilot results show very favorable initial results in many cases as expected or even better. Owing to these successful results, Daleel field is starting a new era in its history with a completely redeveloped horizontal waterflooding pattern of its two major producing blocks with the expectation to get the maximum recovery. In addition to this, the learnings from the pilot tests and the waterflooding experience gained will be the valuable adding to other blocks in Daleel field and other similar fields world-wide. Introduction The Daleel oil field, located in the northwest of the Sultanate of Oman, was discovered in 1984, and came on stream in 1990. The main target reservoir is the Shuaiba carbonate formation, which is characterized by relatively homogeneous properties and a combination of structural and stratigraphic traps. The reservoir units, which cover a surface of 60 km2, are in isolated faulted blocks with low relief anticline elongated in the NE-SW and lithological pinch out in the NW-SE. The porosity ranges from 15% to 35%, with a corresponding permeability range of 4 mD to 20 mD. Although fractures have been detected on FMI/FMS logs acquired from some vertical wells, they seem to have no detectable influence in productivity improvements. The reservoir contains light under-saturated oil (38° API) of 0.85cp initial viscosity and resides in column thicknesses ranging typically from 3 to 20 m. According to the existing fluid PVT reports, the bubble point pressure can be either 1,670 psi or 2,370 psi with initial pressure range of 2,500 psi to 2,607 psi. The vertical depth from the ground is in the range of 1,500 m to 1,610 m. Economical production commenced with vertical wells in depletion mode in 1990. Since 1994 horizontal wells were introduced as producers and got 3,000 bbl/d more production. After 12 years' depletion recovery, the reservoir pressure declined from 2,500 psi to about 900 psi in the most depleted areas. In July 2002, Daleel field was acquired by Daleel Petroleum LLC from Japex Oman. In order to improve the oil recovery, horizontal waterflooding technology was introduced after a detailed field development review. The main waterflooding areas were identified to be the isolated faulted Block-B and Block-C of the Daleel field. The work presented in this paper demonstrates the beginning of a successful horizontal waterflooding experience in a relatively thin, very low-permeability, and highly depleted solution gas-drive carbonate reservoir in Oman. Background of Horizontal Waterflooding and Application in Oman The concept of horizontal waterflooding was introduced by Taber in 1992 as a method of improving the performance of conventional waterfloods. It was pointed out that water can be injected at much higher rates with the lower well head pressures in horizontal wells than in vertical wells, allowing oil to be recovered more quickly. It was further concluded that areal sweep efficiency would be improved by the line-drive geometry as compared to the sweep patterns that develop among vertical wells. Because of the accelerated rate of recovery and the potential for increased the ultimate recovery, it was speculated that horizontal waterflooding could offer significant benefits over conventional waterfloods.