Abstract
Most of drilling hole problems are attributed to wellbore stability issues which adversely cause excessive lost time and cost millions of dollars. The past drilling experiences in Kupal oilfield showed excessive mud losses, kick flows, tight holes and pipe stuck leading to repeated reaming, fishing and sidetracking. Most of the drilling-associated problems in this field occurred during drilling the 12 ¼-in. hole, which is across the non-reservoir Gachsaran formation (consisting of anhydrite, gypsum and marl with thin limestone layers). Mainly due to the lack of required formation evaluation data, no geomechanical studies of this formation have been conducted to date. In this work, first, we constructed a geomechanical model to investigate the root of the problems. This is a pioneer wellbore stability work for such a complex lithology formation which included finding the equations best-matching with core data and field observations. Finally, to overcome the field challenges and hole problems, the study proposes some field remedial actions. The results of the geomechanical modeling show that the pore pressure, shear and tensile failure gradients are greatly variable with the safe mud weight window becoming excessively narrow at some intervals. This accounts for the encountered wellbore stability issues as managing the mud weight in these situations requires several casing strings. To mitigate the extent of the problem, this study proposes the application of innovative drilling technologies including casing while drilling to eliminate the casing running time with potential reduction in drilling time, and continuous circulation system to prevent cuttings settling and kick flows during connections. These technologies are capable of elimination of the geomechanical part of the drilling delay (30% of the average 77 drilling days) per well.
Highlights
Wellbore instability is one of the most critical challenges affecting the well construction phase, but the entire life cycle of a well
Using the geomechanical model and safe mud window developed for Gachsaran formation of well Z-26 (Fig. 13), the field operational challenges are first inferred by considering the safe mud window and field observations of mud loss and kick
In the 2700–2950 m interval of Fig. 13, it is observed that the mud weights are lower than shear failure boundary causing shear failure to occur, which is confirmed by numerous tight holes in this depth interval
Summary
Wellbore instability is one of the most critical challenges affecting the well construction phase, but the entire life cycle of a well. Using the results of the MEM, wellbore stability analysis will be conducted and the geomechanical origin of the borehole problems including the narrow safe mud weight window will be detected This is a pioneer wellbore stability work for a Gachsaran non-reservoir formation (with a complex lithology of anhydrite, gypsum and marl, with thin limestone layers) which is responsible for most of the wellbore stability issues and lost times (NPT). The following subsections briefly review the commonly observed drilling problems (excessive mud losses and tight holes or stuck pipe) as observed in the field as well as indications of low-performance drilling (ROP and NPT). It can potentially cause borehole stability issues (such as kick flows, tight hole, stuck pipe, breakouts and lost circulation), leading to NPT or drilling NPT.
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