Stress related wellbore instability problems in deep wells in ABK field
Authors: Onaisi A. , Locane J. , Razimbaud A. 
Published: December 2007

Problems related to wellbore instability remain a major source of non productive time while drilling, soaring significantly the operational expenditures. Several factors are at the back of wellbore instability, some are the derivative of the regional geology and could not be modified, these as the in situ stresses, the pore pressure and the formation(s) strength(s). Others are the operator's responsibility such as the mud weight, chemistry and flow rate, the casing program and the well trajectory, and might be changed if necessary. More importantly, there are cases where wellbore instability problems could not be fully avoided; improved hole cleaning procedures might be the only way of managing such problems.

The causes and impact on drilling of wellbore instability could be summarized as follows. When a well is drilled, the stresses originally supported by the removed material are redistributed on the rock around. In the post drilling configuration, the deviatoric stress, which is the difference between the major and the minor principal stresses, is maximum at the borehole wall. Plastic rocks creep onto the wellbore, reducing the well diameter. Long periods of reaming/back reaming might be needed while tripping in/out through such geometrical restrictions. Most of rocks, however, display a rather brittle behavior. So, a brittle material tends to fracture as soon as the stress nears its strength. Invasion of the fractures and erosion of the rock fragments by the mud follow the rock disintegration around a wellbore. Beyond a critical well inclination1, big cavings settle forming a bed on the low side. This bed restricts the movements of the drill string increasing the risk of getting stuck.

Four stress regimes prevail in the underground depending on the ratios of the two horizontal principal stresses to the overburden stress. Among these three principal stresses, the maximum horizontal is the most difficult to determine. In the past, methods mostly based on core testing (DSA, ASR, paleo-magnetism) were used to estimate such a stress. More recently, the analysis of the figures of borehole failure, i.e. breakouts and drilling induced fractures, was proposed as a technique to determine the tectonic stress direction and magnitude2,3.

In ABK field, located offshore Abu-Dhabi, no dramatic problems are encountered while drilling into reservoirs belonging to the Thammama and Arab groups (depth TVD &lt; 2300 m), except in the Nahr-Umr shales, the wells being drilled routinely with a mud weight lower than 1.2 SG. At those depths, the stress corresponds to a strike-slip faulting regime with a minimum horizontal stress close to 1.63 SG. Recently, severe wellbore instability troubles occurred while trying to drill horizontal drains in the Hamlah-Gulailah formations with the same mud weight. A rock mechanics study was carried out to identify the causes and find out a remedy to such problems. So far, only two nearly vertical wells could be drilled in the field. Therefore, limited data were available for determining the stresses that included oriented calipers, master logs, pressure and rate records from an acid fracture job and a core.

Several breakouts were identified on the calipers, their sizes vary from small (long axis/well radius &lt; 1.2 and breakout width &lt; 60°) to very large (long axis /well radius &gt; 1.8 and width &gt; 130°). Some intervals are, however, in gauge. A minimum horizontal stress of 2.44 SG was determined using pressure record during an injectivity test realized prior to the acid fracture operation. Triaxial tests were performed on core samples to measure their strengths. Some intervals of the core were fractured, more or less intensively, while others were intact. This explains the strong scattering of the unconfined compressive strengths with values ranging from 55 to 1700 bar, although the most frequent value is 300 bar in average. Most likely, the largest breakouts occur in the softer, most fractured intervals while the strong sections remain perfectly stable. Breakouts dimensions and orientations together with strength values were introduced in the software SFIB©4, which permitted to constrain the magnitude and the direction of the tectonic stress. The stress regime was found to be at the frontier between thrust and strike-slip, with major horizontal stress acting NE-SW. The existence of regional tectonic stresses was confirmed from the observation of breakouts in other fields. Finally, the estimated stress and strength values were used to calculate the optimum mud weight versus well trajectory.

In parallel, DSA tests were performed. The results were not truly reliable. This fact confirms that the techniques of stress measurements based on core testing should be handled with great precaution.