Abstract:
Wellbore instability problems are the cause for the majority of nonproductive time in the southern Iraqi field developments. These drilling difficulties pose potential problems such as mud losses in weak or vugular formations, stuck pipe in shales or unconsolidated formations. This paper focuses on the most severe problem in terms of effort and disbursement which is referred to pipe sticking in Tanuma shale formation. Examining the drilling data revealed that this phenomenon was mostly related to shear failure of the wellbore, which caused an immediate increase in the stand pipe pressure with no drilling fluid returns when circulating. In this regard, a geomechanical analysis for southern Iraqi field was performed on field data from 45 deviated. The analysis identified the following areas of improvement. First, the mud weight being used was not changed properly with respect to variation in wells azimuth and inclination. Secondly, anisotropic effects of this shale formation caused by the bedding planes should be considered in wells trajectory design. Due to the lack of published studies regarding wellbore problems in southern Iraqi fields, this paper could serve as case history for similar fields in that region.
Introduction
Numerous borehole instabilities were observed in the drilling operations of deviated wells through shaly formations in southern Iraqi oil and gas fields. These problems usually escalate during increased inclination of the wells, resulting in significant nonproductive time. Drilling data from various deviated wells designed to drill the productive interval of limestone (Mishrif formation) in different inclinations and azimuths from different slots is shown in Figure 1. Various problems have been occurred; varying from, stuck pipe, mud losses, sulfurous water flowing, hole tightening, and extensive caving. The stuck pipe problems caused wells to be drilled with more than one side tracks or loss of the hole in worst case. Whereas other problems could considerably increase the nonproductive time or well control issues for severe mud losses. Therefore, the diagnosis of the root cause of borehole failure is a necessary procedure in the well constructing and planning phase. Another possible type of problem is the collapse failure of the borehole which happens when the formation around the vicinity of the wellbore wall fails due to either shear or tensile failure (Fjaer, 2008). Consequently, the wellbore will be tightened or packed off if the drilling cuttings are inadequately transported out of the hole (due to insufficient mud properties, flow rate or string rotation). Additionally, the stuck pipe may occur when the plastic region around the borehole is unexpectedly yielded in the case of insufficiently supported by the borehole pressure (Tare et al., 2002). However, if the drilling fluid density is greater than the weakest formation fracture gradient then the drilling fluid will be lost, and fracture treatments should be performed. Besides, the high difference between wellbore pressure and formation pressure could lead to differential sticking incidents. Pipe sticking is the most severe problem with the majority of the nonproductive time. Borehole shear failure could be one of the potential reasons for this drilling instability.
This paper will focus on the geomechanical analysis of wellbore instabilities of the Tanuma shale in southern Iraqi fields. The study aims to address the minimum required mud weight in order to prevent the onset of shear failure wellbore for different well paths.
