The need to increase productivity reducing drilling formationrndamage favours the use of underbalanced drilling technology.rnThe main idea is to drill with equivalent circulating densitiesrn(ECD) below the formation pore pressure and to avoid therncontact between the drilling fluid and the formation. In highlyrndepleted reservoirs, pore pressures can be very low. Therefore,rnextremely low density fluids such as foams are used to achieverncirculating densities below pore pressure. In such cases, therninduced modification of the in situ stresses has to be supportedrnmainly by the rock, with low contribution of the drilling fluidrnpressure. In that sense, the application of underbalancedrndrilling depends, among other factors, on the mechanicalrnstability of the drilled formation. In general, poorlyrnconsolidated depleted formations are not suited for thatrntechnology. This paper presents the wellbore stabilityrnsimulation performed in order to establish the feasibility ofrnusing underbalanced drilling in highly depleted reservoirs inrnwestern Venezuela. The in situ stress field and the mechanicalrnproperties of the formation were obtained. Pore pressure asrnlow as 800 psi at 5500 ft (2.7 lb/gal equivalent fluid density)rnwas measured. The finite difference method and anrnelastoplastic constitutive model was used to obtain the newrnstress, deformation and pore pressure distribution. Thernundrained condition (immediately after the wellbore is drilled)rnas well as the drained condition were analysed. The analysisrnshowed that horizontal wells could be drilled in anrnunderbalanced condition with low instability risk. Followingrnthe recommendations, four horizontal wells were drilled inrnunderbalanced conditions. Values as low as 2.0 lb/gal ECDrnwere used to drill the wells and no wellbore instabilityrnproblems were reported. Production tests showed an enormousrnincrement in well productivity index in comparison withrnconventional overbalanced drilling.
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机译：需要提高生产率以减少钻井地层损伤，这有利于使用欠平衡钻井技术。主要思想是以低于地层孔隙压力的等效循环密度（ECD）进行钻井，并避免钻井液与地层之间的接触。在高度贫乏的油藏中，孔隙压力可能非常低。因此，极低密度的流体（例如泡沫）被用来实现低于孔隙压力的循环密度。在这种情况下，原位应力的诱导变化必须主要由岩石来支撑，而钻井液压力的贡献很小。从这个意义上讲，欠平衡钻探的应用除其他因素外，还取决于钻探地层的机械稳定性。通常，固结不佳的枯竭地层不适合该技术。本文介绍了进行井眼稳定性模拟的方法，目的是为了确定在委内瑞拉西部高耗竭油藏中利用欠平衡钻井的可行性。获得了地层应力场和地层的力学性能。测量了在5500英尺（2.7磅/加仑等效流体密度）rn时低至800 psi的孔隙压力。用有限差分法和弹塑性本构模型求出新的应力，变形和孔隙压力分布。分析了排水不畅的情况（钻井后立即）以及排水情况。分析表明，水平井可以在失衡条件下进行钻探，具有较低的失稳风险。根据建议，在不平衡条件下钻了四个水平井。低至2.0 lb / gal ECD的值被用来钻井，没有井眼失稳的问题的报道。生产测试表明，与常规的超平衡钻井相比，油井生产率指数大幅提高。
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