致密气藏压裂直井导流能力对有效缝长的影响

发布时间：2018-08-26 15:39

【摘要】：开发致密气藏,生产井通常需要经过水力压裂才能实现工业化开采。通过泵入大量混有支撑剂的压裂液到地层中,形成一条导流能力较高的支撑裂缝,改善气体流入井筒的渗流通道,从而提高单井产能和最终采收率。但是,由于各种原因,低渗气井产能远远小于我们的预测产能。通过大量研究发现最重要的原因就是有效缝长小于设计缝长。形成这种短缝的原因是缝高失控、裂缝端部脱砂、支撑剂分布不合理、支撑剂回流,也有可能是受它们的综合影响。当然,这也有可能是因为压裂液破胶不彻底而导致压裂液返排太慢。众所周知,当压裂施工结束后,仍然有部分压裂液残留在裂缝和地层当中,降低了侵入带地层的有效渗透率从而导致气井产能的降低。因此,为了提高压裂效果,必须保证地层中尽可能少的压裂液残余量。然而,裂缝导流能力与有效缝长的影响关系还不是很明确。因此,研究导流能力与有效缝长的关系,给出合理的导流能力和有效缝长,对后期压裂优化设计,实现致密气藏高效开发具有重大意义。在压裂设计中,我们一般将支撑缝长(设计缝长)定义为有效缝长。然而,致密气藏普遍具有超低含水饱和度现象,压后在裂缝附近地层易形成水相圈闭区,阻碍了气体向裂缝中的流动,真实的有效缝长也就小于设计缝长。本文中将有效缝长定义为对气井产能有实际贡献的裂缝长度(有气体流入的裂缝长度)。本文对中江沙溪庙组的静动态地质特征进行分析,结果表明该区直井低产的主要原因是侵入带气体相对渗透率较低、有效缝长小于设计缝长。对该区S1井进行加砂压裂,一周后进行压力恢复测试,测试结果表明该井外推地层压力为47.6252MPa,地压系数1.68,与原始地层压力相近。然而,分析得出的地层有效渗透率只有0.00468×10-3μm2,远小于实际值0.18×10-3μm2,有效缝长只有17m,为设计缝长的8.9%,更加验证了致密砂岩储层水相圈闭区域的存在。压裂液的侵入增加了裂缝侵入带的含水饱和度,从而降低了气体的相对渗透率,过短的有效缝长更是导致了实际产能低于预测产能。然而,致密砂岩储层普遍具有超低含水饱和度现象(实际含水饱和度低于最小含水饱和度),目前我们无法完全消除水相圈闭对致密气藏的伤害,因此我们只能尽可能的增加压裂液的返排来最大限度的增加有效缝长,提高气体产能。我们借助油藏数值模拟技术,通过建立气水两相二维模型来研究在不同地层渗透率、不同裂缝半长、不同滤失量及不同裂缝侵入带伤害程度四种条件下裂缝导流能力对有效缝长的影响。通过气体流入裂缝累计分布曲线图和多种导流能力条件下不同生产时间点的有效缝长对比图来分析气体沿支撑裂缝的分布情况以及各种影响因素下,导流能力对有效缝长的影响。研究表明,研究区沙溪庙组具有较好的地质资源和良好的勘探开发前景。压裂液的侵入对于裂缝侵入带气体的相对渗透率造成了重大伤害。开采初期,有效缝长远小于设计缝长,随着生产时间的增加有效缝长在1年后基本都能接近设计缝长。尽管气体沿着整条裂缝流动,但是残留在裂缝侵入带中的压裂液不可能完全返排出来。含水饱和度的分布影响着气体向裂缝的流动速度,甚至决定有效裂缝的长度。气体流动效率的轻微的改变将对有效裂缝长度的变化起着非常大的影响。换而言之,裂缝导流能力越大,压裂液返排就更加彻底,有效缝长就会更长,增产效果就会更加明显。
[Abstract]:In the development of tight gas reservoirs, production wells usually need hydraulic fracturing to realize industrial production. A high conductivity supporting fracture is formed by pumping a large amount of fracturing fluid mixed with proppant into the formation, which improves the percolation channel of gas into the wellbore, thereby increasing the single well productivity and ultimate recovery. The productivity of low permeability gas wells is far less than our predicted productivity.The most important reason is that the effective fracture length is less than the designed fracture length.The causes of this kind of short fracture are uncontrolled fracture height,desanding at the end of the fracture,unreasonable proppant distribution,proppant reflux and their comprehensive influence. It is well known that some fracturing fluids remain in fractures and formations after fracturing operation, which reduces the effective permeability of the formation in the intrusive zone and leads to the decrease of gas well productivity. Therefore, in order to improve the fracturing effect, it is necessary to ensure as little as possible in the formation. However, the relationship between fracture conductivity and effective fracture length is not very clear. Therefore, it is of great significance to study the relationship between fracture conductivity and effective fracture length, and to give reasonable conductivity and effective fracture length for later fracturing optimization design and efficient development of tight gas reservoirs. Supporting fracture length (designed fracture length) is defined as effective fracture length. However, compact gas reservoirs generally have ultra-low water saturation. Water-phase traps are easily formed near fractures after compression, which hinder gas flow into fractures, and the actual effective fracture length is less than the designed fracture length. In this paper, the static and dynamic geological characteristics of Shaximiao Formation in Zhongjiang are analyzed. The results show that the main reason for low production of vertical wells in this area is that the relative permeability of gas in the intrusion zone is low and the effective fracture length is less than the designed fracture length. The test results show that the extrapolated formation pressure is 47.6252 MPa and the ground pressure coefficient is 1.68, which is similar to the original formation pressure. However, the analysis shows that the effective permeability of the formation is only 0.00468 6550 The invasion of fracturing fluid increases the water saturation of the fractured intrusive zone, which reduces the relative permeability of the gas. The short effective fracture length leads to the fact that the actual productivity is lower than the predicted productivity. We can not completely eliminate the damage of water trap to tight gas reservoir before, so we can only maximize the effective fracture length and improve gas productivity by increasing fracturing fluid flowback as much as possible. The influence of fracture conductivity on effective fracture length under four conditions, i. e. different filtration rate and different damage degree of fracture invasion zone. The gas distribution along supporting fracture and various influencing factors are analyzed by the cumulative distribution curve of gas inflow fracture and the effective fracture length contrast chart of different production time points under various flow conductivity conditions. The results show that the Shaximiao Formation in the study area has good geological resources and good prospects for exploration and development. The invasion of fracturing fluid causes great damage to the relative permeability of gas in the fracture-invaded zone. Although gas flows along the entire fracture, the remaining fracturing fluid in the fracture-invading zone can not be completely discharged. The distribution of water saturation affects the velocity of gas flowing into the fracture, and even determines the length of the effective fracture. In other words, the greater the fracture conductivity, the more thorough the fracturing fluid backflow, the longer the effective fracture length, and the more obvious the stimulation effect.
【学位授予单位】：成都理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TE377

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