Rate of penetration (ROP) enhancement in shales through osmotic processes
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Shales and other clay-bearing rocks make up 70% to 80% of all formations drilled globally. In addition, according to the US Energy Information Administration, as of June 2013, approximately 26% of the technically recoverable hydrocarbon resources of the United States were in the form of shale tight oil and shale gas. Accordingly, there has been an increasing focus on efficient drilling of deep and tight shale formations. However, serious drilling problems such as bit balling and wellbore instability are encountered while drilling shale, especially in deep, high-pressure wells. Most of these drilling problems are not caused by the mechanical strength of the rock but by the chemically reactive nature of such formations, causing rock cuttings to stick to the bit. Such an undesirable effect decreases the rate of penetration (ROP) and has a damaging effect on the wellbore. Previous approaches to this problem were based on either improving the drilling hydraulics or the polycrystalline diamond compact (PDC) bit design, or modifying the mud chemistry. This thesis introduces a new approach to prevent adherence of shale cuttings to the bit by taking advantage of the phenomenon of chemical osmosis. Chemical osmosis occurs when fluid movement is governed by chemical potential gradients. Drilling with a high salinity/ low water activity drilling mud gives inception to movement of water out of shale cuttings toward the drilling bit due to the difference of water activity between the shale and the drilling fluid. While previous studies showed that the movement of water out of shales also helps to mechanically stabilize them, this study suggests that such a dehydration of formation rock cuttings also prevents shale cuttings from sticking to the bit, resulting in higher rates of penetration. It is also suggested that the same mechanism is responsible for achieving the ROP benefits from high salinity formate muds, which enabled three times faster drilling than the oil-based mud at high densities during the deep trek testing in 2002. Support for the proposed mechanism is provided by an extensive set of laboratory measurements, ranging from simple hot-rolling tests that measure accretion tendencies of shale on steel to realistic drilling tests on full-scale equipment and actual PDC bits under downhole conditions. Results from the drilling tests indicate that both low and high salinity formate muds outperform the corresponding weight water-based muds in the Mancos shale, by yielding 50% to 60% higher ROP especially at a higher weight on bit (WOB; i.e., corresponding to greater depths). Results from field trials with formate muds for ROP enhancement in Canada indicate significantly enhanced drilling rates with respect to those of oil/synthetic mud causing an approximately 50% reduction in average drilling time, thereby supporting the merits of chemical osmosis for ROP enhancement.