利用元素伽马能谱测井识别气层的数值模拟

Numerical simulation on gas reservoir identification using element gamma spectrum well logging

利用蒙特卡罗方法建立不同井眼和地层条件下的计算模型,模拟研究非弹俘获计数比R与含气饱和度的测井响应规律及井筒和地层环境对R的影响;利用研究成果处理新疆某油田实测井资料,验证元素伽马能谱测井技术在实际生产应用进行地层含气性评价的可行性。结果表明:当地层孔隙度一定时,非弹和俘获伽马计数随着含气饱和度的增加而增加,而比值R减小;当孔隙度增大时,含气饱和度小于某一确定值(约35%)时非弹伽马计数减小,但大于该值时俘获伽马计数减少,比值R增加;岩性、地层水矿化度、井眼流体和尺寸以及钻井液侵入等因素都会对地层非弹和俘获计数比产生影响;不同井眼流体对非弹俘获计数比R的影响较小,在同一地层孔隙度条件下,地层水矿化度越高、井径越大,非弹俘获计数比R越大,且石灰岩地层的非弹俘获计数比R高于砂岩地层;钻井液侵入较浅时,非弹俘获计数比R随钻井液侵入的加深而增大,当达到探测深度后,侵入加深但非弹俘获计数比R保持不变;地层水矿化度越高、井径越大、钻井液侵入越小越有利于含气饱和度的评价,但在实际处理时需要对岩性、地层水矿化度、井径进行校正。

Using Monte Carlo method, we build simulation models under different borehole and formation conditions to simulate the response of R (the ratio of inelastic gamma counts and capture gamma counts) and gas saturation.Meanwhile, the influence of R under different borehole and formation environment conditions are also simulated.This method is further applied to a gas well in Xinjiang Oil field to evaluate gas saturation. The results validate the effectiveness of this method. The simulation results show that both inelastic and capture gamma counts increase in the same porosity, while the value of R decreases. When formation porosity increases, inelastic gamma counts decrease while gas saturation is less than a certain value (about 35%). The trendis opposite when gas saturation is greater, capture gamma counts decrease while the value of R increases.Various factors, such as lithology, formation water salinity, borehole fluid types, borehole size and drilling mud invasion can affect the accuracy of R . Therefore, these factors should be corrected. The effect of the borehole fluid types is not significant. When formation porosity is confirmed, the higher of formation water salinity and the bigger of the borehole diameter are, the greater value of R becomes. In addition, R is larger in limestone formation than in sandstone formation. When the depth of mud invasion is less than the detection depth, the value of R increases with the increasing mud invasion depth. High salinity formation water, big borehole size and deep mud invasion are beneficial for gas saturation evaluation.