Understanding borehole stability has become increasingly important in recent years to facilitate design of costly horizontal wells. This paper presents a case study, in which we evaluated the stability of a horizontal well in a major California oil field by first determining rock strength and tectonic stress and then drilling the well to verify the results of the predictions. Given the opportunity to drill numerous horizontal and possibly multilateral wells, it was important to fully evaluate the possibility of a successful open hole completion. An 800-foot horizontal well was planned to be drilled within a poorly consolidated and highly porous Pliocene reservoir that is slightly over pressured.
The analysis revealed that one horizontal principal stress is greater and one is lower than the overburden. Maximum horizontal compression is oriented N60°E. Borehole stability of the horizontal well is predominantly controlled by the extremely weak reservoir rock (uniaxial compressive strength is only 1,175 psi). An open hole completion was predicted to collapse without supporting slotted liner or casing. Even for a cased well, perforations placed at their most stable orientation were predicted to produce considerable amounts of sand. Only 200 feet of open horizontal hole was tested to validate the prediction. The open hole produced briefly and then collapsed.
The horizontal well was subsequently completed in 800 feet of interval using a 60 mesh slotted liner and has produced 15,657 bbls of oil, 291 bbls of water, and 41,181 Mcf of gas through 12/31/99 with no sand problems.
The drilling process changes the in situ stress in the near wellbore region by developing a stress concentration at the wellbore wall1,2 (hoop stress). The rock surrounding the hole must support the stress previously carried by the material removed. Because the magnitudes of the in situ principal stresses are generally different (i.e., the vertical stress, Sv, and the two horizontal stresses, SHmax and Shmin, are all unequal)3–5, the magnitude of the stress concentration varies markedly with azimuth around the well1,6,7. Furthermore, the wellbore stress concentration depends on the magnitudes and orientations of the principal in situ stresses8–10. Therefore, deviation and azimuth of a wellbore are also critical for the hoop stress magnitude developing around the borehole wall.
Wellbore stability is controlled by the interplay between the stress and rock strength at any given depth. The borehole will fail when the stress concentration around the borehole wall exceeds the strength of the rock. Failures can occur in compression (resulting in wellbore breakouts2,11) or in tension (resulting in tensile wall fractures12–15) and it is important to note that they can occur without loss of the well. Because these failures occur at azimuths that are a function of the stress orientation and magnitudes and deviation of the wellbore15,16, they can be utilized when detected in the wellbore to determine the stress field13,14,17–19. Once this stress field is known for a particular reservoir, it can be utilized to evaluate and predict stability of new wells of arbitrary trajectory.
We illustrate the approach of wellbore stability evaluation in a case study, where an 800-foot long horizontal well was planned to be drilled in a poorly consolidated and highly porous Pliocene reservoir of a major California oil field. A possible method of optimizing exploitation of the reservoir requires a substantial program of horizontal drilling. Two questions needed to be answered in this case study. First, is it feasible to complete open hole for the proposed wellpath? The trajectory strikes parallel to the anticline that defines the structure of the field. Second, does the well need to be cased and perforated in order to withstand the increased effective stresses associated with production-related drawdown?