川东地区井喷显示特征及地质因素分析

SIGNS AND GEOLOGIC FACTORS OF BLOWOUT IN GAS WELLS IN EAST SICHUAN BASIN

川东地区是目前四川盆地天然气勘探开发的主战场,钻井工作量大,钻井中气侵、井涌、溢流显示频繁,特别是个别井喷事故的发生给企业带来了严重的经济损失和不良的社会影响。为此,通过对川东天然气井喷特征及其地质因素的分析,以期预防或减少井喷事故的发生。研究表明:①川东碳酸盐岩地层发生井喷的频率较高,但在1990年后井喷次数已明显减少;②所发生的主要是钻进中井内为高负压条件下的井喷,喷出的天然气中硫化氢含量高,危害性大;③绝大多数的井喷在发生前都有明显征兆。地质因素分析结果认为:①低缓、潜伏构造及部分高陡构造上异常高压裂缝系统,特别是小型网状或单裂缝系统井喷频率高;②在纵向压力剖面上,嘉二2亚段以下异常高压带井喷的频率高,地层层位上以嘉二1亚段—嘉一段、飞仙关组、茅二段井喷为主;③多个地层压力带或复杂压力系统共存于同一裸眼井段的条件下,易喷、漏共存。

Eastern Sichuan Basin is the focus of gas exploration and development in the Sichuan Basin at present, with numerous wells being drilled and to be drilled. Signs of gas invasion, gas kick and overflow occur frequently during drilling operation, and some serious blowout accidents have caused large economic loss and negative social influences to oil companies. It is expected to prevent or reduce blowout accidents through analyses of the signs and geologic factors of blowout in gas wells in eastern Sichuan Basin. The following understandings are achieved through study: ① The frequency of well blowout is relatively high in the carbonates in East Sichuan Basin, but has been lowering significantly since 1990; ② Most of the blowouts occur at high negative borehole pressure during drilling operation and the natural gas blowing out has high content of hydrogen sulfide, thus is highly pernicious; ③ Most of the well blowouts show obvious signs before they occur. The following conclusions are drawn from analyses of the geologic factors: ① The frequency of blowout is high in intervals with abnormally high pressure fracture system, especially small map cracking or single fracture system, in gentlely buried structures and some highly steep structures; ② On vertical pressure profile, the frequency of blowout is high in the abnormally high pressure zones below T1j22B, while in respect of horizons, blowouts mostly occur in T1j21B-T1j11, T1f, and P1m2; ③ Blowout and lost circulation may occur concurrently in an open hole interval with multiple formation pressure zones or complex pressure systems coexisted.