Algeria Block 350 is located in the northwest of the Oued Mva sub-basin in the Sahara Desert with an area of 8666km2. From 1972 to 2007, CFPA, Talisman, SSRP, Sonatrach, and CNPCIAE drilled in succession 10 wells in the block. However, all the wells failed to reach an industrial oil and gas production rate due to the high mud content in the tight and low-permeable sandstone reservoirs. Though the hydrocarbon detectors of two wells, Bel-E-1 and Meg-2, which were drilled by CNPC, showed the gas measurement values up to 900,000 ppm, the DST testing results were still far from expectation. In order to eliminate the impact of reservoir pollution caused by drilling fluids, series of stimulation operations such as pickling and large-scale fracturing (calculated crack length 75 meters) were conducted to well Bel-E-1 after it was cased. Nevertheless, the testing results only received 4.8 m3/d of oil and 5800 m3/d of gas, which were still below the threshold for economical benefit. By analyzing existing well data and testing results, experts believe that it is difficult to make an exploration breakthrough by using conventional drilling technology in such a reservoir with poor physical properties, so some other solutions are in need. After more than six months of in-depth research and repeated demonstration, CNPC decided to use underbalanced drilling technology to tackle the block's exploration problems, which has been proved entirely correct finally. On July 25, 2008, the underbalanced horizontal well Bel-Z-1 was tested with 24/64"(9.5mm) choke for the Devonian Siegenian layer, and a result of 9.71×104 m3 of gas and 15 m3 of condensate oil achieved under the WHP of 1314psi. This paper reviews challenges encountered in designing and constructing the well Bel-Z-1 and related solutions so as to share our experience with the industry.

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