深水高温高压环境下钻井全过程井壁稳定评价方法

Several special problem are faced when drilling in deepwater ,such as the peculiarity of geology and offshore environment, the strong plasticity in shallow formation,the complexity of temperature field in wellbore and the narrow drilling fluid density window.So to maintain the stability of wellbore in whole process of deepwater drilling wall is related to the key to deepwater drilling safety. The preliminary study did not determine the deformation law and constitutive equation of the seabed sediments, and did not consider the effect of transient surge pressure on the stability of the borehole. Therefore, the method of wellbore stability evaluation in the whole process of deep well under HTHP is still a scientific problem to be solved. This paper focuses on the key scientific problems and the key scientific problems that affect the instability of the wellbore in the deepwater HTHP environment. The rock samples under different seabed shallow soil conditions and water content are consolidated and the circular three-axis test is conducted under the condition of undrained drainage.through the test ,the deformation and distribution of the shallow sediments is revealed and the influence of formation penetration, high temperature and mud circulation on formation collapse pressure and fracture pressure is Determined .establish a evaluation method of the whole drilling process under HTHP environment, combined with the "stress cloud" and the Drucker-Prager criterion determine the influence of the fluctuating pressure on the stability of the borehole wall. Basis on this, the method of wellbore evaluation of the whole process of deep well under high temperature and high pressure environment is established , which provide theoretical basis for the efficient and safe development of deepwater oil and gas resources in the South China Sea.

深水钻井面临着特殊地质环境，浅部地层塑性强，井筒温压场复杂，钻井作业窗口窄，保持深水钻井全过程井壁稳定是关系到深水钻井安全的关键。前期研究未明确海底沉积层的变形规律及本构方程、未考虑波动压力对井壁稳定的影响，深水钻井全过程井壁稳定评价方法仍是有待解决的科学问题。本课题重点针对深水高温高压环境下影响井壁失稳的敏感因素及评价方法这一关键科学问题，对不同海底浅层土质条件及含水率条件下的岩样进行固结不排水条件下的室内循环三轴试验，揭示深水浅层沉积层的变形分布规律；以地层非稳态导热为基础，确定地层渗透、高温和泥浆循环对地层坍塌压力、破裂压力的影响规律；建立波动压力诱导井周应力及孔隙压力改变的计算模型，结合井周“应力云”与Drucker-Prager准则确定波动压力对井壁稳定的影响规律；在此基础上建立深水高温高压环境下钻井全过程井壁稳定评价方法，为实现我国南海深水油气资源的高效安全开发提供理论依据。

深水浅部地层欠压实程度高，具有弱胶结、承压能力低等力学特性，安全密度窗口狭窄，保持钻井过程中的井壁稳定难度大。井筒温压场复杂、井筒深部地层温度较高，交变温度使钻井液流变性变化较大，地层温度与钻井液之间的热交换会产生附加应力场，导致坍塌压力、破裂压力的变化，同时，由于深水钻井泥浆密度窗口窄，起下钻过程中产生的波动压力诱导井周应力变化，从而导致常规的井壁稳定分析方法难以满足深水钻井安全的需要，给深水钻井井筒压力控制带来挑战。本项目针对以上问题，主要研究成果如下：.（1）建立了深水浅部地层井壁失稳评价方法，为深水浅层安全钻井提供模型参考，也为深水钻井浅部地层固井提供一定的理论依据；.（2）建立了井筒在钻井液循环作用下高温地层温度分布规律，综合考虑地应力、孔隙压力、地层孔隙弹性、循环注替过程中井壁温度变化附加应 力和井壁渗流等因素，分析井周应力状态；结合地层强度破坏准则，建立地层坍塌压力、破裂压力计算模型，形成深水环境下的长距离、大温差井段井筒温度变化引起的井壁稳定评价方法。 .（3）评估了深水起下钻过程中钻柱震动对瞬时波动压力的影响，建立波动压力诱导井周应力及孔隙压力改变的计算模型，并将使用分析模型研究随时间变化的井筒稳定性。