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 elastic 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 m(3)/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 m(2) (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 extensive 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 m(3)/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.