高级检索

煤储层改造背景下深部煤层气藏物质平衡方程的建立与应用

Establishment and application of material balance equation for deep coalbed methane reservoirs based on coal formation stimulation background

  • 摘要: 近些年深部煤层气发展迅速,在多个深部煤层气区块都取得单井日产万方的突破,为了更加科学高效地开发深部煤层气,大幅提高煤层气的采收率,建立合理、可靠且考虑因素全面的煤层气物质平衡方程,并研究深部煤层气藏或气井控制储量、平均煤储层压力、不同类型气体产量占比、产能评价指标和最终可采储量具有重要的理论与现实意义。目前考虑游离气、煤基质收缩、溶解气、吸附气富集的微孔与游离气和水占据的介孔之间的压差和煤储层改造影响的煤层气物质平衡方程鲜有报道。首先基于深部煤层气藏物质平衡原理,在常规物质平衡方程考虑应力敏感引起的孔隙压缩、基质收缩、水的膨胀和产水因素的基础上,进一步考虑吸附气富集的微孔与游离气和水占据的介孔之间的压差、煤储层改造引起的煤储层物性参数变化和溶解气的影响,建立了基于煤储层改造背景下的深部煤层气藏物质平衡方程;然后提出了深部煤层气线性拟合的吸附气−游离气−溶解气储量评价方法、平均地层压力显式计算方法、吸附气−游离气−溶解气产量占比评价方法、深部煤层气井产能评价方法和最终可采储量EUR预测方法;最后对提出的方法进行了实例应用,评价了实例井控制的吸附气−游离气−溶解气储量,揭示了深部煤层气井生产过程中吸附气−游离气−溶解气产量占比变化规律,剖析了产能指标变化特征并预测了不同废弃压力下的EUR和煤层气采收率。研究表明:提出的深部煤层气储量评价方法,仅需实测2次以上的平均煤储层压力和对应的累计产气量和产水量数据,即可采用线性拟合的方法,评价出深部煤层气井吸附气−游离气−溶解气的储量;提出的深部煤层气平均地层压力计算方法为显式表达式,避免了隐式求解方法中复杂的计算机编程计算;提出的深部煤层气井吸附气−游离气−溶解气产量占比评价方法,无需在井口安装碳同位素监测装置,可实时评价出不同气体产量占比;提出的综合流体、游离气、吸附气、溶解气和水井底采出指数可以用于识别深部煤层气井工作制度的合理性;提出的深部煤层气井EUR预测方法,可将生产数据整理成视压力p/Z*与累计产气量Gp的直线表达式,给定某一废弃压力,即可预测出深部煤层气井的EUR;实例井生产初期游离气日产气量占比和吸附气日产气量占比相当,在生产过程中,游离气日产气量占比先快速上升后缓慢降低最后逐渐稳定在29%,吸附气日产气量占比则先快速降低后缓慢上升最后逐渐稳定在70%,而溶解气日产气量占比一直较低;当废弃压力分别为4、3和2 MPa时,该深部煤层气井的煤层气采收率分别为37.8%、44.2%和52.4%,降低废弃压力是提高深部煤层气藏采收率较为有效的手段。

     

    Abstract: In recent years, the deep coalbed methane was developed rapidly, with breakthroughs in daily production of tens of thousands of cubic meters per well in multiple deep coalbed methane blocks. In order to develop deep coalbed methane more scientifically and efficiently, significantly improve the recovery rate of coalbed methane, establishing a reasonable, reliable, and comprehensive coalbed methane material balance equation, and studying the controlled reserves, average coal reservoir pressure, proportion of different types of gas production, productivity evaluation indicators, and EUR of deep coalbed methane reservoirs or gas wells have important theoretical and practical significance. At present, there are few reports on the material balance equation of coalbed methane considering the presence of free gas, coal matrix shrinkage, the presence of dissolved gas, the pressure difference between micropores enriched with adsorption gas and mesopores occupied by free gas and water, and the impact of coal formation stimulation. Firstly, based on the principle of material balance in deep coalbed methane reservoirs, a material balance equation for deep coalbed methane reservoirs under formation stimulation background was established by further considering the pressure difference between micropores enriched with adsorption gas and mesopores occupied by free gas and water, changes in coal reservoir physical parameters caused by formation stimulation, and dissolved gas, except considering pore compression caused by stress sensitivity, matrix shrinkage, water expansion, and water production in conventional material balance equation of coalbed methane reservoir. Then, a linear fitting method for evaluating the reserves of adsorbed gas, free gas, and dissolved gas, an explicit calculation method for average formation pressure, an evaluation method for the production proportion of adsorbed gas, free gas, and dissolved gas, a productivity evaluation method for deep coalbed methane wells, and an EUR prediction method were proposed. Finally, the proposed methods were applied in an example to evaluate the reserves of adsorbed gas, free gas, and dissolved gas controlled by the well. The variation law of the production proportions of adsorbed gas, free gas, and dissolved gas in the production process of deep coalbed methane well were revealed, and the characteristics of changes in productivity indicators were analyzed. The EUR and coalbed methane recovery under different abandoned pressures were predicted. Results show that: The proposed method for evaluating coalbed methane reserves only requires two or more actual measurements of the average coal reservoir pressure and corresponding cumulative gas and water production data, and can use linear fitting to evaluate the reserves of adsorbed gas, free gas, and dissolved gas controlled by deep coalbed methane wells. The proposed method for calculating the average formation pressure of deep coalbed methane is an explicit expression, which avoids the complex computer programming calculation of implicit solving methods; The proposed evaluation method for the production proportion of adsorbed gas, free gas, and dissolved gas in deep coalbed methane wells does not require the installation of carbon isotope monitoring devices at the wellhead, and can real-time evaluate the proportion of different gas production. The proposed productivity indexes of comprehensive fluid, free gas, adsorbed gas, dissolved gas, and water at bottomhole condition for deep coalbed methane wells can be used to identify the rationality of working systems of the deep coalbed methane well; The proposed EUR prediction method for deep coalbed methane wells can organize production data into a linear expression of apparent pressure p/Z * and Gp. Given a certain abandoned pressure, the EUR of deep coalbed methane wells can be predicted. In the initial stage of production of the example well, the production proportion of free gas is almost equivalent to that of adsorbed gas. During the production process, the proportion of free gas first rapidly increases, then slowly decreases, and finally gradually stabilizes at 29%. The production proportion of adsorbed gas first rapidly decreases, then slowly increases, and finally gradually stabilizes at 70%, while the production proportion of dissolved gas remains low. When the abandoned pressure is 4, 3, and 2 MPa, the coalbed methane recovery rates of the deep coalbed methane well are 37.8%, 44.2%, and 52.4%, respectively. Reducing the abandoned pressure is an effective way to improve the coalbed methane recovery rate of the deep coalbed methane reservoirs.

     

/

返回文章
返回