页岩气吸附解吸效应对基质物性影响特征

Influences of adsorption/desorption of shale gas on the apparent properties of matrix pores

为了研究页岩气降压开采过程中吸附气解吸作用对基质表观物性(如有效孔隙半径、有效孔隙度、表观渗透率)及气体流动机制的影响,推导了吸附解吸作用下页岩基质孔隙有效半径和表观渗透率动态模型,建立了考虑吸附解吸影响基质表观物性和气体传输机制的页岩气渗流数学模型。采用有限体积法对模型进行求解,利用实验及矿场数据验证了模型的可靠性,最后应用该模型研究了页岩气开采过程中基质物性参数、气体流动机制变化特征以及吸附效应对页岩气开发的影响规律。研究结果表明,页岩气开采过程中基质孔隙有效半径、有效孔隙度和表观渗透率逐渐变大,体积压裂改造区域流动机制由滑脱流转变为过渡流;忽略吸附层影响将导致地质储量和产气量严重高估;随着吸附层厚度增加,累计产气量变化不大,但采收率逐渐降低。

The adsorption/desorption effects impact the petro-physical properties of matrix pores during gas depressurizing production, such as effective pore radius, effective porosity, apparent permeability, which will further impact the gas flow regime. In this paper, the dynamic models of effective pore radius and apparent permeability under the action of gas adsorption/desorption are derived. The mathematical model of shale gas flow is established considering the effects of adsorbed gas on apparent properties and gas flow regime. After that, the model is solved and validated using a finite volume method and experimental and field data. Finally, the variations of apparent parameters of matrix pores and gas flow regimes during gas production, and the influences of adsorption on gas production are demonstrated. The results show that the effective pore radius, porosity and apparent permeability increase during gas production; the gas flow regime in stimulated reservoir volume （SRV） changes from slip flow to transition flow; if the impacts of adsorbed gas on gas production is overlooked, both original gas in place （OGIP） and gas production will be significantly overestimated. The cumulative gas production changes slightly as the adsorption layer thickness increases, but the gas recovery factor decreases.