Geology Department, California State University Sacramento , Sacramento, California 95819, United States.
National Energy Technology Laboratory, U.S. Department of Energy , Pittsburgh, Pennsylvania 15236, United States.
Environ Sci Technol. 2017 Aug 15;51(16):9391-9402. doi: 10.1021/acs.est.7b01979. Epub 2017 Aug 2.
Hydraulic fracturing for gas production is now ubiquitous in shale plays, but relatively little is known about shale-hydraulic fracturing fluid (HFF) reactions within the reservoir. To investigate reactions during the shut-in period of hydraulic fracturing, experiments were conducted flowing different HFFs through fractured Marcellus shale cores at reservoir temperature and pressure (66 °C, 20 MPa) for one week. Results indicate HFFs with hydrochloric acid cause substantial dissolution of carbonate minerals, as expected, increasing effective fracture volume (fracture volume + near-fracture matrix porosity) by 56-65%. HFFs with reused produced water composition cause precipitation of secondary minerals, particularly barite, decreasing effective fracture volume by 1-3%. Barite precipitation occurs despite the presence of antiscalants in experiments with and without shale contact and is driven in part by addition of dissolved sulfate from the decomposition of persulfate breakers in HFF at reservoir conditions. The overall effect of mineral changes on the reservoir has yet to be quantified, but the significant amount of barite scale formed by HFFs with reused produced water composition could reduce effective fracture volume. Further study is required to extrapolate experimental results to reservoir-scale and to explore the effect that mineral changes from HFF interaction with shale might have on gas production.
水力压裂技术现已广泛应用于页岩气开采中，但对于储层中页岩-水力压裂液（HFF）的反应却知之甚少。为了研究水力压裂停泵期间的反应，实验在储层温度和压力（66°C，20 MPa）下，让不同的 HFF 通过压裂后的马塞勒斯页岩岩心流动一周。结果表明，含有盐酸的 HFF 会导致碳酸盐矿物的大量溶解，这与预期一致，使有效裂缝体积（裂缝体积+近裂缝基质孔隙度）增加了 56-65%。含有再利用采出水成分的 HFF 会导致次生矿物，特别是重晶石的沉淀，使有效裂缝体积减少 1-3%。尽管实验中存在防垢剂，且有或没有页岩接触，但重晶石的沉淀仍会发生，这部分是由 HFF 中过硫酸盐破胶剂在储层条件下分解产生的溶解硫酸盐驱动的。矿物变化对储层的总体影响尚未量化，但再利用采出水成分的 HFF 形成的大量重晶石垢可能会降低有效裂缝体积。需要进一步研究以将实验结果外推到储层尺度，并探讨 HFF 与页岩相互作用引起的矿物变化对天然气产量的影响。
