Practical Approach to Achieve Accuracy in Sanding Prediction

et al. 2006

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper described a case study involved an investigation in a field in Libya, where massive unexplained fill had been reported accompanying obstruction of production for majority of production wells since the onset of production, indicating possible sanding issues for this field.To investigate this problem, relevant data from different sources and different domains (i.e., wireline logs, laboratory test data, drilling data, well data and field data) were integrated to generate a Mechanical Earth Model (MEM). This model provided the descriptions of the rock strengths and insitu stresses in the reservoir formation. Somewhat surprisingly, the model, backed up by the core laboratory test data, observations from core inspection and thin section analyses, revealed the rocks to be extremely hard and strong, and therefore highly unlikely to sand. These findings contradicted with initial impression and previous expectation on this sandstone that it should have been sand-prone formation. Facing these apparent inconsistencies, the investigation moved beyond an initial focus of sanding risk evaluation and sandface completion optimization. The final results revealed that the problems facing the field were other than conventional sanding and formation failure, and that they involved some rather interesting and misleading phenomena, such as precipitation of salt from production, tubing scale, spalling of borehole wall with drawdown and cavings bridge (cavings might fall into and become wedged in the openhole, forming a bridge with no material beneath).The investigation concluded that installing sand control facilities were unnecessary, which otherwise would have cost millions of dollars without correctly addressing the real problem that this field was facing. The study highlighted the importance of a thorough investigation of the mechanism and source of sanding rather than premature conclusions based initial, and potentially misleading evidence. It also highlighted how the integration of information from different sources and disciplines were able to correctly identify and address a particular borehole fill problem, allowing for optimizing field operations, field management and workover strategies.

Sanding—Not as it First Appeared

Aboulsayen¹,

Elnaami²,

et al. 2006

Underbalanced Drilling Of a Horizontal Well in Depleted Reservoir: A Wellbore Stability Perspective

Gerryo²,

Tan³

et al. 2007

The need to reduce formation damage and to avoid differential sticking and lost circulation in depleted reservoirs favors the use of underbalanced drilling (UBD) technology. In highly depleted reservoirs, the pore pressure can be very low, necessitating the use of extremely low-density fluid to achieve an equivalent circulating density (ECD) below the pore pressure. In such situations, the stress redistribution around the wellbore has to be supported mainly by the rock matrix, and limited support is provided by the mud pressure. Therefore, UBD can dramatically increase the risk of wellbore instability. From literature review, it was found that the techniques and methodologies to carry out wellbore stability analysis on UBD of horizontal wells are not well documented. This paper presents a wellbore stability study that was conducted to evaluate the feasibility of using UBD technology to drill a horizontal well in a highly depleted reservoir in Libya. The study started with geomechanical laboratory tests and Mechanical Earth Model (MEM) construction to evaluate the in-situ stresses, pore pressure, and mechanical properties of the formations likely to be encountered during the planned UBD campaign. Wellbore stability analysis was subsequently conducted for the planned horizontal well to be drilled underbalance using a new practical approach. The analysis revealed a high probability of extensive and severe breakout within the weak zones if penetrated underbalanced and the potential for massive wellbore collapse. Under the guidance of this study, the well plan and drilling designs were amended to minimize drilling risk. Introduction There is a heavily depleted field in the Sirte Basin in Libya with reservoir pressure around 2,200 psi (equivalent to 4.9 lbm/gal fluid density) that is depleted from an initial reservoir pressure of around 3,800 psi in the year of 1969. Accompanying the reservoir pressure depletion, the production of the field dramatically declined. To rejuvenate this field, new horizontal wells are planned both as producers and injectors. As the reservoir pressure is so low, underbalanced drilling (UBD) technology was initially recommended to reduce formation damage and to avoid mud loss and differential sticking during drilling.1-7 Additionally, possible benefits derived from UBD include an increase in rate of penetration (ROP), an increase of bit life, and early production of hydrocarbons. However, UBD could exacerbate potential wellbore failure in comparison with conventional overbalance drilling as the mud pressure support on the borehole wall is removed.8 The potential for excessive wellbore failure during drilling could render UBD technology unfeasible. To evaluate the potential wellbore instability risk with UBD, a wellbore stability study was initiated. The implementation of the UBD technology would be decided based on the wellbore stability analysis results. Towards this end, a data audit was conducted on two offset wells (Well A and Well B) included in the study of the field. Consistency of these data was checked and errors or inaccuracies detected in the data were corrected. Additionally, we performed a thorough review of drilling events encountered during drilling the two wells to gather information that would be used to identify and characterize the drilling problems. Geomechanical laboratory tests were carried out to obtain accurate descriptions of rock mechanical properties. By using these audited data and geomechanical laboratory test data, we generated a MEM for Well B for which complete suites of data were available. The model was subsequently validated by comparing predicted wellbore instabilities based on the model with borehole failure observations. The MEM for Well A was built subsequently by reusing the stress model developed from the construction of the MEM for Well B. Once the MEM of Well A was validated, it was propagated to a planned horizontal trajectory in the locality of the well. Wellbore stability analysis was conducted for the horizontal well to be drilled underbalanced. A Drill Map was created for the planned horizontal well as the integrated outcome of wellbore stability analysis.

Geomechanics Enables the Success of Horizontal Well Drilling in Libya: A Case Study

Felgueroso²,

Lalinde³

et al. 2008

Drilling highly deviated or horizontal wells can be prone to instability problems. This paper describes a case in Libya on which significant difficulties were encountered during drilling the first horizontal development well in a field in Murzuq basin. The first two branches of the well were lost due to severe instability problems. A comprehensive geomechanics study was carried out to understand the causes of the wellbore failure and to improve drilling design and drilling performance on further development wells in the field. The study specifically included:Performing a systematic data audit and integration to identify specific uncertainties in the input data and identify data gaps.Performing a comprehensive drilling event review to investigate what happened during drilling and what were the major instability problems. All drilling events were organized, categorized and input into a database. A graphical presentation of the drilling events review was generated.Applying a fit-for-purpose stability model. This simple model is driven by easily accessible input parameters and provides a rapid answer.Validating a Mechanical Earth Model through history matching. This procedure reduced the uncertainty of the model and the resulting wellbore stability predictions.Applying innovative visualization and presentation of wellbore stability analysis. This improves understanding and awareness of the various drilling hazards and enables effective utilization of the wellbore stability information during drilling. The analysis identified the cause of wellbore instability, as being inadequate mud weight while drilling the overlying shale formation in the deviation build-up section. The design of the second horizontal well was optimized based on this study. The well was drilled successfully without problems and, in fact, ahead of drilling schedule. This case demonstrated that a comprehensive geomechanics analysis can greatly improve drilling performance and reduce drilling costs. Introduction Horizontal wells can increase production rates and ultimate recovery, and reduce the number of platforms or wells required to develop a reservoir. The geometry also helps to delay water or gas breakthrough, bypass environmentally sensitive areas and reduce stimulation costs.1 To achieve avoidance of water coning and delay of water breakthrough, Akakus Oil Operations started to drill the first horizontal development well H1 in a field in Libya in 2006. However, unexpected drilling difficulties were encountered and the first two branches of the well were lost. Prior to this project, quite a significant number of exploration and development vertical wells had been drilled in the same block without experiencing any major problem. A wellbore stability study was carried out to understand the cause of wellbore failure in the horizontal well, to optimize the drilling design and performance for the next horizontal development wells to be drilled in the same field.