In a drilling/production operating environment, wellbore instabilities arise in all three stages of a wells life span: drilling, stimulation and production phases. The timing and severity of the occurrence of such borehole problems dictate which method of stability analysis should be used. Pseudo-3D codes are plane-strain, non-isotropic subsets of full 3D numerical and analytical codes. Their speed, portability, and ease of use have popularized them among operations engineers. There are numerous versions of pseudo-3D stress analysis, from simple linear-elastic to sophisticated poro-elastic-plastic, each with its own advantage that suits a particular wellbore problem. Simple linear elastic codes are re-emerging in popularity because of ease of use and field-calibration schemes.
Recent technological advances are pushing the reach of boreholes beyond 25,000 ft in length. Highly inclined, extended-reach wellbores (ERD) must remain open for prolonged time-periods, not only during the drilling program but also in the life of a reservoir. In a commercial operating environment, the technical staff must perform long-range planning to avoid potential drilling and production hazards, and still be able to quickly solve occurrences of unplanned wellbore instabilities. This paper presents strategies in analyzing various wellbore stress problems encountered in day-to-day field activities.
There are three stages in the life of a well:
1. drilling,
2. completion and stimulation, and
3. flow tests, production, and depletion.
These different uses and stages in the life of a well should dictate which method of stability analysis is appropriate and applicable. The usual problem is to find the feasible/acceptable limits of the wellbore pressure Pw during all the three phases. Although good drilling and completion plans include rock mechanical analysis, in any field operation, unforeseen instabilities may still arise, and the timing of such events may also dictate the manner in which the stability analysis is performed.
This stage warrants an integrated stability analysis because it is the most capital-intensive. A review of recent advances in drilling ERD wells is given by Payne, Wilton and Ramos (1995). The main concern is to determine the mud composition and density which will maintain the integrity of the well, without the loss of drilling fluids. Conventionally, the choice of mud density or well pressure Pw is dictated by the highest formation pore pressure Pr along the well path, Figure 1. Other operational and geological factors listed below must be considered: Abnormally pressured layers, depleted zones Fractured formations, loss of circulation zones Penetration rate, mud / clay compositions Differential sticking, kicks and blow-outs Coring recovery, cementing efficiency Formation damage, logging, well tests Commonly encountered instabilities and their presumed mechanisms of failure are listed in Table 1, assuming that mud chemistry has already been optimized. Instability simply means that at some point, the rock shear strength or tensile strength has been exceeded, and its severity ranges from negligible to collapse. Major wellbore collapse problems are shear-failure induced, but in case of loss of drilling circulation, the mechanism is tensile failure.
