Horizontal Water Disposal Well Performance in a High Porosity and Permeability Reservoir
Authors: Harding T.G. , Smith K.H. , Norris B. 
Published: March 2010

Nexen Petroleum International Ltd. (Nexen) has a 52% interest and is operator of the Masila Block in the Republic of Yemen. Oil and water are produced mainly from the under pressured Qishn Formation, a non-marine to marine clastic sequence of Lower Cretaceous Age, which is roughly 61 m (200 ft) thick and lies at a depth of 191 m (5,500 ft) from surface. Currently, oil production is 36,567 m3/d (230,000 BOPD) at a water cut of about 80%. The 160,000 m3/d (1.0 MMBWPD) of water produced are currently reinjected under matrix injection pressures into 24 vertical and four horizontal wells. These are completed in the best quality sands (the S2/S3 members of the Upper Qishn Formation) that have average porosity and permeability of 20% and 3.65E-12 m2 (3,700 md), respectively.

Despite the exceptional disposal reservoir quality, injection problems continue to exist that have caused Nexen to study and evaluate numerous methods of improving injectivity. After xtensive laboratory core and field testing, hypotheses have been developed to explain the behaviour of the water disposal wells including the so-called "check valve effect." Horizontal wells and proppant fractured wells were employed to test the hypotheses and to improve injectivity.

This paper reviews the laboratory results and discusses the placement of horizontal injectors along with the drilling and completion details of the wells. The performance of the horizontal disposal wells under matrix injection is compared to conventional vertical disposal wells and proppant fractured vertical wells. Produced water is expected to reach 238,500 m3/d (1.5 MMBWPD) and improvements in disposal well performance will reduce the number of wells that will need to be drilled to handle this volume, thereby improving overall project value.

Introduction

The Masila Block in the Republic of Yemen is operated by Nexen Petroleum International Ltd. on behalf of its partners: Occidental Petroleum Ltd., Consolidated Contractors Company S.A.L., and the Government of the Republic of Yemen. Oil production from the block began in 1993 and current field oil, gas, and water production rates average 36,567 m3/d (230,000 BOPD), 200,000 m3/d (7 MMSCFD), and 160,000 m3/d (1,000,000 BWPD), respectively. Figure 1 shows the location of the 15 main producing fi elds, the central processing facility (CPF) and the hydrocyclone installations at main producing fields. These hydrocyclones are used to achieve initial separation of oil from water at the individual pools remote from the CPF. The operation of the Masila project is described in an earlier paper with particular attention paid to disposal of produced water(1).

The majority of oil produced to date has been from the Upper Qishn Formation, which is supported by a strong natural water drive. Other producing horizons include the Lower Qishn clastics, Upper Saar clastics, Saar carbonate, Madbi limestone, Basal sand, and the fractured granitic Basement (Figure 2). Most produced water is currently reinjected into the S3 and S2 members of the Upper Qishn Formation, and it is therefore worthwhile to review the geology of these two members.

The S3, at the base of the Upper Qishn Formation, is an amalgamated fluvial channel sequence.