Underbalanced Drilling of a Horizontal Well in Depleted Reservoir: A Wellbore-Stability Perspective
- Kaibin Qiu (Schlumberger) | Yousef Gerryo (AGOCO) | Chee P. Tan (Schlumberger WTA Malaysia S/B) | James R. Marsden (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2008
- Document Type
- Journal Paper
- 159 - 167
- 2008. Society of Petroleum Engineers
- 3 Production and Well Operations, 5.1.10 Reservoir Geomechanics, 5.3.2 Multiphase Flow, 5.6.9 Production Forecasting, 1.2.1 Wellbore integrity, 1.6 Drilling Operations, 1.8 Formation Damage, 1.6.7 Geosteering / Reservoir Navigation, 1.2.2 Geomechanics, 1.2.3 Rock properties, 2.4.3 Sand/Solids Control, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.7.1 Underbalanced Drilling, 1.1 Well Planning, 5.5 Reservoir Simulation, 5.5.8 History Matching, 1.7 Pressure Management
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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 that 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 underbalanced 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.
There is a heavily depleted field in the Sirte basin in Libya, with reservoir pressure approximately 2,200 psi (equivalent to 4.9-lbm/gal fluid density) that is depleted from an initial reservoir pressure of approximately 3,800 psi in 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.
Because the reservoir pressure is so low, UBD technology was initially recommended to reduce formation damage and to avoid mud loss and differential sticking during drilling (Bennion and Thomas 1994; Falk and McDonald 1995; Doane et al. 1996; Robinson et al. 2000; Sarssam, et al. 2003; Qutob 2004). 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-overbalanced drilling because the mud pressure support on the borehole wall is removed (Hawkes et al. 2002). 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 on the basis of wellbore-stability-analysis results.
Toward this end, a data audit was conducted on two offset wells (Well A and Well B) included in the study of the field. The consistency of these data was checked, and errors or inaccuracies detected in the data were corrected. Additionally, a thorough review of drilling events encountered during drilling of 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 an MEM for Well B for which complete suites of data were available. The model was subsequently validated by comparing predicted wellbore instabilities on the basis of 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.
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