Resumen del autor, extraído del trabajo. Canadian Occidental Petroleum Ltd. (CanadianOxy) has a 52 percent interest and is operator of the Masila Block in the Republic of Yemen. Oil and water are produced mainly from the underpressured Qishn sandstones of Lower Cretaceous age, which lie at a depth of 5500 to 6000 feet from surface. The sands are highly porous, permeable (1,000 to 25,000 millidarcy), and very loosely consolidated. Oil production is from fifteen main fields within an area of 484 square miles. Total field oil, gas, and water production rates in early 2000 average 215,000 bopd, 7 mmscfd, and 700,000 bwpd, respectively, from 120 active production wells. Plans are in place to handle produced water volumes growth to 1,100,000 bwpd. Produced water is currently reinjected into 21 wells completed in the producing Qishn reservoirs. All produced water will continue to be reinjected into the producing formations. Typical water disposal wells exhibit the so-called 'check-valve' effect where injectivity index is very much lower than productivity index. This summary discusses investigation of the check-valve effect, the difficulties and challenges that have been encountered as well as solutions found to the disposal of large volumes of water. An overview of laboratory work, theoretical analyses, geological and geomechanical aspects, well completion methods, field trials and water quality issues are discussed and their effects on the "check valve" effect.