水基压裂液对致密砂岩气层的损害机理——基于《水基压裂液性能评价方法:SY/T 5107—2016》的改进

Damage mechanism of water-based fracturing fluid to tight sandstone gas reservoirs: Improvement of The Evaluation Measurement for properties of Water-based Fracturing Fluid: SY/T 5107-2016

目前,针对压裂液的损害评价主要参考《水基压裂液性能评价方法 :SY/T 5107—2016》(以下简称行业标准),而行业标准设定的压裂液驱替岩心过程与实际压裂施工中压裂液高压快速侵入储层的过程不吻合,并且压裂液损害评价实验未考虑压裂液残渣、破胶方式、原始含水饱和度等因素的影响,从而影响了评价结果准确性。为此,选取塔里木盆地库车坳陷迪北地区下侏罗统阿合组(J1a)致密砂岩岩心,通过改进实验流程与方法来模拟压裂液侵入致密砂岩气层的过程,评价了压裂液对气层的损害程度,进而系统剖析了损害机理。研究结果表明:①改进后的压裂液损害评价方法,考虑了气层的原始含水饱和度(小于束缚水饱和度)、压裂过程中的高压瞬间"击穿"效应、压裂液残渣等因素的影响,能更加客观地评价压裂液对致密砂岩气层的损害程度;②基于行业标准的评价结果显示,压裂液对致密砂岩气层渗透率的损害程度为中等偏强,而改进后方法得到的损害程度则为中等偏弱;③压裂液残渣滞留于裂缝中是对渗透率造成损害的主要因素,残渣易堵塞裂缝与缝面孔,绝大多数残渣滞留在基质岩心表层(侵入深度小于3 cm)孔隙中,基质孔隙对残渣有过滤作用;④压裂液从岩心表面向内部运移的过程中,高分子聚合物依次以薄膜层状、局部片团状、晶体包裹状滞留于储层孔隙中;⑤实验条件下,岩心中会产生盐析晶体,盐析晶体在岩心中分布不均,裂缝中盐析晶体与高分子聚合形成复合包裹体,基质孔隙中盐析晶体与少量伊利石等碎片包裹形成复合体;⑥速敏产生的运移微粒通常与残渣、高分子聚合物等形成复合包裹体,进而堵塞裂缝与孔隙。

At present, evaluation on reservoir damage induced by fracturing fluid mainly refers to The Evaluation Measurement for Properties of Water-based Fracturing Fluid: SY/T5107-2016(referred to as the industry standard below). However, the fracturing fluid displacing core process stipulated in the industry standard is not consistent with the fast invading process of fracturing fluid into the reservoir under high pressure during the actual fracturing construction. besides, the influences of fracturing fluid residue, gel breaking mode, original water saturation and other factors are not taken into consideration in the experiments to evaluate the damage of fracturing fluids. Thus, the accuracy of evaluation results is influenced. In this paper, tight sandstone cores of the Lower Jurassic Ahe Formation(J1 a) in Dibei area of Kuqa Depression of the Tarim basin were selected as samples. The invading process of fracturing fluid into a tight sandstone reservoir was simulated by modifying experimental process and method. Then, the damage degree of fracturing fluid to gas reservoir was evaluated and the damage mechanisms of fracturing fluid were analyzed systematically. And the following research results were obtained. First, the modified evaluation method takes into account the influences of several factors, such as the original water saturation of gas reservoir, the instantaneous "breakdown" effect of high pressure during fracturing and the fracturing fluid residue, so it can evaluate the damage degree of fracturing fluid to tight sandstone gas reservoirs more objectively. Second, the evaluation results based on the industry standard show that the damage degree of fracturing fluid to the permeability of tight sandstone gas reservoirs is medium to strong, whereas the damage degree evaluated by the modified method is medium to weak. Third, the retention of fracturing fluid residue in fractures is the main cause of permeability damage. The residues can easily block fractures and fracture surface pores. Most of them retain in the pores in the surface layer of matrix cores(invasion depth less than 3 cm), so residues are filtered by matrix pores. Fourth, when fracturing fluid migrates inwards from the core surface, high molecular polymers retain in the form of thin-film lamellar, local flaky nodular and crystal inclusion in turn in the reservoir pores. Fifth, under the experimental conditions, salting-out crystals appear and are unevenly distributed in the cores. In fractures, salting-out crystals and high molecules are polymerized to form composite inclusions. In matrix pores, salting-out crystals and a small number of fragments(e.g. illite) are enclosed to form a complex. Sixth, migratory particles caused by speed sensitivity are usually combined with residues and high molecular polymers to form composite inclusions, thus blocking pores and fractures.