超高压定容气藏弹性能量变化特征

Elastic energy variation in volumetric gas reservoirs with ultrahigh pressure

超高压定容气藏的原始地层压力高、初期产能高,驱动能量以天然气弹性驱动为主;较之常压气藏,超高压气藏在开发过程中的压力变化区间大,有必要在开发初期即对气藏弹性能量变化开展预测,以便掌握压力变化规律及制定合理采气速度。为此,基于物质平衡方程,定义了气藏弹性能量指数(Eg)、气藏弹性能量系数[f(p)],推导了表征超高压定容气藏弹性能量随压力变化的表达式,分析了影响因素,并采用该方法计算了铜锣峡长兴组气藏的弹性能量变化,进一步验证其可靠性。研究结果表明:①等温开发过程中,气藏Eg主要受气藏压力和储量影响。②超高压定容气藏的f(p)随压力变化表现为凸曲线,当气藏压力从超高压开始下降时,弹性能量系数随压力降低而逐渐增大,至压力降低为11 MPa左右时,弹性能量系数达到最大,之后随压力降低而减小,总体上低压阶段的弹性能量系数是超高压阶段的3倍。③不同组分干气气藏的弹性能量系数差别小,即甲烷含量对弹性能量系数的影响小。④对于铜锣峡气田长兴组气藏这类超高压定容气藏,气藏开发存在压力、产量与稳产时间的固有矛盾,对策为开发初期应尽量控制井数和单井产量,延长气藏在高压区间的生产时间,降低由于初期配产过高而带来的不利影响;在气藏压力降低后,应适当增加井数、提高采气速度,使得整个开发周期内采气速度保持相对稳定。

With the elastic drive as an driving energy in natural gas,most of volumetric gas reservoirs with ultrahigh pressure are featured by high initial formation pressure and productivity.They varies greatly in pressure during their development compared with the other ones with normal pressure.It is necessary to predict the elastic energy variation in these ultrahigh-pressure gas reservoirs at the early development stage so as to make clear pressure change laws and formulate a reasonable gas production rate.Therefore,on the basis of material balance equation,both elastic energy index(Eg)and elastic energy coefficient[f(p)]were defined,one expression representing the change of elastic energy with pressure was deduced for the volumetric gas reservoirs,and influential factors were analyzed.Furthermore,the elastic energy coefficient in gas reservoirs of Changxing Formation in Tongluoxia gasfield was calculated by means of this method to reflect the energy variation,further verify method’s reliability.Results show that(1)during the isothermal development,Eg is mainly affected by both pressure and reserves;(2)f(p)in the volumetric gas reservoirs with ultrahigh pressure assumes a convex curve with pressure change.The elastic energy coefficient may gradually increase with pressure drop when pressure starts to drop from an ultrahigh one.It tops the highest when the pressure falls to about 11 MPa,and then decreases with pressure drop.Generally,this coefficient in the low-pressure stage is triple that in the ultrahigh-pressure stage;(3)the coefficient differs slightly in dry gas reservoirs with different components,namely a little effect of methane content on this coefficient;and(4)for such volumetric gas reservoirs with ultrahigh pressure as Changxing Formation in Tongluoxia gasfield,there exists an intrinsic contradiction among pressure,production and stable production time during their development.To resolve this contradiction,some countermeasures have been made,including to control well quantity and single well production if possible in the early development stage,to prolong exploitation time while reservoirs still at high pressure in order to reduce a certain adverse effect inferred from extra-high initial proration,and to moderately add well quantity and gas production rate after reservoir pressure declines for the sake of comparatively stationary rate throughout the development.