Exploration drilling in carbonate reservoirs has encountered a variety of drilling challenges due to high stresses, overpressure and risk of significant losses due to fractured rocks. In this study, borehole instability is assessed in several wells in a fractured carbonate play. A comprehensive formation evaluation program in two initial wells included borehole image data, highdefinition spectroscopy data and core from multiple sections. In-situ stresses were constrained using borehole image data with special attention paid to presence and absence of drilling-induced tensile fractures, as well as compressive failure and rock strength test results. The spectroscopy data were used to help explain and constrain borehole stability differences observed in different wellbores which was not as readily detected in standard log suites. A subsequent third wellbore was drilled in which the mud weight program attempted to account for possible over pressure and weaker clay-rich zones while at the same time avoiding high losses. The new mud weight program allowed for significant borehole breakouts to occur in order to decrease mud losses; losses were half the amount compared to the previous well over the same challenging formations.


Chevron has recently undertaken exploration drilling for tight carbonate reservoirs. A number of drilling challenges exist in the field including overpressure, high horizontal stresses, weaker shale zones and significant losses due to the fracture natured of the rock. Multiple formations are targeted and good hole condition is required for optimal formation evaluation and cementing operations.

Two previous wells in the field used different static mud weight (MW) and equivalent circulating density (ECD) and as a result had different hole conditions and amount of losses. Well 1 had higher MW and ECD, breakouts and some washouts but significant losses while Well 2 had lower MW and ECD, severe enlargements but minimal losses (Fig. 1). In particular in Well 2, the caliper data across the weaker shale zones were extended to the maximum tool capacity over a large extent of the 17-½” section. The challenge for the upcoming wells is determining a MW which will reduce the breakouts and associated non-productive time (NPT) from borehole instability without causing severe losses and the associated NPT with curing the losses. In this high stress environment, it may be that inclined wells will provide higher drilling margins (i.e., greater difference between collapse and losses pressure) than vertical wells.

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