电缆地层测试评价井旁裂缝的有限元数值模拟方法

本文采用有限元方法模拟了电缆地层测试器的双封隔器和谐波脉冲测试方法在井旁裂缝中的压力响应.根据裂缝性储层渗流力学原理,首先模拟了双封隔器压力测试在裂缝与井壁相交和不相交两种情况下的压力响应,模拟结果表明:当裂缝与井壁相交时,压力响应随裂缝导流性质的变化发生显著改变;当裂缝与井壁不相交时,除非裂缝的导流能力非常大或离井壁非常近,否则压力响应随着裂缝导流能力的变化并不明显.说明双封隔器测试方法可以有效评价与井壁相交裂缝的导流能力,而对远离井壁的裂缝并不敏感.谐波脉冲压力测试一直被用来探测地层的各向异性,本文通过数值模拟方法探讨和分析了谐波测试方法探测和评价与井壁不相交裂缝的可行性,数值模拟结果表明谐波的压力幅度和相位延迟对裂缝的导流能力变化敏感,说明该方法可以用来评价井旁裂缝.此外,本文还对脉冲频率和双探针间距对评价效果的影响进行了分析.

Finite element studies on characterization of near wellbore fractures using the wireline formation tester

Fractures play an important role in both reservoir fluid flow and well productivity. Fracture conductivity is an important parameter to evaluate the fractured reservoir. It is difficult to evaluate the fracture conductivity with conventional prospecting methods. The wireline formation tester has been applied to evaluate the fracture conductivity near wellbore. The effects of evaluating the fractures crossed and non-crossed well borehole with the method of interval pressure transient tests and harmonic pulse testing are analyzed.#br#In order to research the effects that evaluate the fractures in the reservoir with the wireline formation tester, we used the finite element method to simulate the pressure response of fractured reservoirs with the interval pressure transient tests and harmonic pulse testing. At first, we simulated the pressure response of different fracture parameters with interval pressure transient tests and analyzed the different effects of evaluating the fractures crossed and non-crossed well borehole. Then we analyzed the feasibility of evaluating the fractures non-crossed well borehole with the harmonic pulse testing. The pressure response of different pulse frequencies and different dual probe spacing were simulated and the simulation results of different fracture parameters were compared and analyzed.#br#In order to investigate the effect of the fracture conductivity on the interval test behavior, at first, we changed the fracture conductivity in the range from 1×10^-15m³, 1×10^-14m³, 1×10^-13m³ to 5×10^-13m³. The simulation results indicate that pressure drawdown is faster and buildup occurs slower as the fracture conductivity decreases. The pressure differential and MDH curves of different fracture conductivity values can be distinguished easily from a pressure gauge. It presents that the fracture conductivity can be evaluated by the interval pressure transient tests efficiently. In the second case, the fractures do not intersect with the well borehole. The fracture conductivity varies from 1×10^-17m³, 1×10^-15m³, 1×10^-14m³ to 1×10^-12m³. The simulation results show that pressure responses have a small change when the fracture conductivity is varied. Although the values of different pressure differentials can be distinguished by a quartz gauge, the effect on pressure response is negligible. Then we used Harmonic pulse testing to evaluate non-crossed fractures. At first, the pulse frequency was defined as 0.1 Hz, the spacing of dual probe was defined as 20 cm. From the simulation results, there is a certain pressure differential and phase shift between the fractured reservoir and non-fractured reservoir. In addition, the pressure differential and phase shift can be distinguished between different fractures conductivity values. When the spacing of the dual probe was equal to 50 cm, there is evident amplitude differential and phase shift between fractured formation and non-fractured formation. Thus it indicates the long spacing of dual probe can detect those fractures near the well borehole more efficiently. Finally, we simulated the pressure response for different radial depths of fractures near well borehole, for which the pulse frequency was defined as 0.1 Hz and the spacing of the dual probe was 50 cm. According to the simulation results, the method of harmonic pulse testing can detect the fractures near the well borehole efficiently. However, it becomes more difficult and probably invalid for detecting those fractures located far away from the well borehole.#br#The fractures intersecting the well borehole can be evaluated efficiently by interval pressure transient tests depending on fracture conductivities. Non-intersecting fractures in the vicinity of the well borehole have negligible effects with the method. However, when the fracture conductivity increases, or the fractures get close to the well borehole, the effect becomes noticeable. Harmonic pulse testing can detect and evaluate the non-crossed fractures with appropriate spacing of the dual probe and pulse frequency. Through decreasing pulse frequency and increasing dual probe spacing, it can increase detection distance and improve the detection precision. However, this method becomes much less precise when the fractures conductivity become enough large or fractures located far away from the well borehole.