A Multidisciplinary Approach to In-Situ Stress Determination and Its Application to Wellbore Stability Analysis

To prevent or minimise wellbore instability problems, it is critical to determine the optimum wellbore profile and to design an appropriate mud weight program based on wellbore stability analysis. It is a complex and iterative decisionmaking procedure since various factors, such as in-situ stress regime, material strength and poroelastic properties, strength and poroelastic anisotropies, initial and induced pore pressures, must be considered in the assessment and determination.This paper describes the methodology and procedure for determination of optimum wellbore profile and mud weight program based on rock mechanics consideration. The methodology is presented in the form of guideline charts and the procedure of applying the methodology is described. The application of the methodology and procedure is demonstrated through two field case studies with different in-situ stress regimes in Australia and Indonesia.

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Transient Coupling of Swab/Surge Pressure and In-Situ Stress for Wellbore-Stability Evaluation During Tripping

SPE Journal ◽

10.2118/180307-pa ◽

2018 ◽

Vol 23 (04) ◽

pp. 1019-1038 ◽

Cited By ~ 3

Author(s):

Feifei Zhang ◽

Yongfeng Kang ◽

Zhaoyang Wang ◽

Stefan Miska ◽

Mengjiao Yu ◽

...

Keyword(s):

Stability Analysis ◽

Pore Pressure ◽

In Situ Stress ◽

Wellbore Stability ◽

Pressure Oscillations ◽

Deepwater Drilling ◽

Wellbore Pressure ◽

Operating Window ◽

Safe Operating

Summary This paper identifies wellbore-stability concerns caused by transient swab/surge pressures during deepwater-drilling tripping and reaming operations. Wellbore-stability analysis that couples transient swab/surge wellbore-pressure oscillations and in-situ-stress field oscillations in the near-wellbore (NWB) zone in deepwater drilling is presented. A transient swab/surge model is developed by considering drillstring components, wellbore structure, formation elasticity, pipe elasticity, fluid compressibility, fluid rheology, and the flow between wellbore and formation. Real-time pressure oscillations during tripping/reaming are obtained. On the basis of geomechanical principles, in-situ stress around the wellbore is calculated by coupling transient wellbore pressure with swab/surge pressure, pore pressure, and original formation-stress status to perform wellbore-stability analysis. By applying the breakout failure and wellbore-fracture failure in the analysis, a work flow is proposed to obtain the safe-operating window for tripping and reaming processes. On the basis of this study, it is determined that the safe drilling-operation window for wellbore stability consists of more than just fluid density. The oscillation magnitude of transient wellbore pressure can be larger than the frictional pressure loss during the normal-circulation process. With the effect of swab/surge pressure, the safe-operating window can become narrower than expected. The induced pore pressure decreases monotonically as the radial distance increases, and it is limited only to the NWB region and dissipates within one to two hole diameters away from the wellbore. This study provides insight into the integration of wellbore-stability analysis and transient swab/surge-pressure analysis, which is discussed rarely in the literature. It indicates that tripping-induced transient-stress and pore-pressure changes can place important impacts on the effective-stress clouds for the NWB region, which affect the wellbore-stability status significantly.

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A Novel Method to Determine the Drilling Fluid Density for Gypsum-Salt Layer

10.2118/208099-ms ◽

2021 ◽

Author(s):

Jitong Liu ◽

Wanjun Li ◽

Haiqiu Zhou ◽

Yixin Gu ◽

Fuhua Jiang ◽

...

Keyword(s):

Drilling Fluid ◽

In Situ Stress ◽

Wellbore Stability ◽

Shrinkage Rate ◽

Fluid Density ◽

Salt Layer ◽

Rock Creep ◽

Key Factor ◽

Well Abandonment

Abstract The reservoir underneath the salt bed usually has high formation pressure and large production rate. However, downhole complexities such as wellbore shrinkage, stuck pipe, casing deformation and brine crystallization prone to occur in the drilling and completion of the salt bed. The drilling safety is affected and may lead to the failure of drilling to the target reservoir. The drilling fluid density is the key factor to maintain the salt bed’s wellbore stability. The in-situ stress of the composite salt bed (gypsum-salt -gypsum-salt-gypsum) is usually uneven distributed. Creep deformation and wellbore shrinkage affect each other within layers. The wellbore stability is difficult to maintain. Limited theorical reference existed for drilling fluid density selection to mitigate the borehole shrinkage in the composite gypsum-salt layers. This paper established a composite gypsum-salt model based on the rock mechanism and experiments, and a safe-drilling density selection layout is formed to solve the borehole shrinkage problem. This study provides fundamental basis for drilling fluid density selection for gypsum-salt layers. The experiment results show that, with the same drilling fluid density, the borehole shrinkage rate of the minimum horizontal in-situ stress azimuth is higher than that of the maximum horizontal in-situ stress azimuth. However, the borehole shrinkage rate of the gypsum layer is higher than salt layer. The hydration expansion of the gypsum is the dominant reason for the shrinkage of the composite salt-gypsum layer. In order to mitigate the borehole diameter reduction, the drilling fluid density is determined that can lower the creep rate less than 0.001, as a result, the borehole shrinkage of salt-gypsum layer is slowed. At the same time, it is necessary to improve the salinity, filter loss and plugging ability of the drilling fluid to inhibit the creep of the soft shale formation. The research results provide technical support for the safe drilling of composite salt-gypsum layers. This achievement has been applied to 135 wells in the Amu Darya, which completely solved the of wellbore shrinkage problem caused by salt rock creep. Complexities such as stuck string and well abandonment due to high-pressure brine crystallization are eliminated. The drilling cycle is shortened by 21% and the drilling costs is reduced by 15%.

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