Natural Fracture Identification and Characterization While Drilling Underbalanced
- Jack Hunter Norbeck (Colorado School of Mines) | Ernesto Rafael Fonseca (Shell Oil Co.) | D.V. Griffiths (Colorado School of Mines) | Sau-wai Wong (Shell Int E P)
- Document ID
- Society of Petroleum Engineers
- SPE Americas Unconventional Resources Conference, 5-7 June, Pittsburgh, Pennsylvania USA
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 3 Production and Well Operations, 1.12.3 Mud logging / Surface Measurements, 2 Well Completion, 1.12.6 Drilling Data Management and Standards, 5.6.1 Open hole/cased hole log analysis, 1.7.1 Underbalanced Drilling, 5.1.2 Faults and Fracture Characterisation, 3.3.2 Borehole Imaging and Wellbore Seismic, 4.1.2 Separation and Treating, 4.1.9 Tanks and storage systems, 5.1 Reservoir Characterisation, 5.8.6 Naturally Fractured Reservoir, 5.6.4 Drillstem/Well Testing, 4.3.4 Scale, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.8.2 Shale Gas, 1.6.6 Directional Drilling, 5.8.1 Tight Gas, 1.5 Drill Bits, 1.6 Drilling Operations
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An algorithm based on data obtained during underbalanced drilling operations is described for identifying the location and properties of productive natural fractures that are intersected by the wellbore. The criteria used to identify natural fracture locations are (i) total gas concentration measurements from mud logs and (ii) mud pit volume. The paper also describes a simple approach for estimating the fracture permeability. The algorithm is applied to data from three sets of horizontal parallel wells with spacing of roughly 700 ft. In two of the three well pairs considered, conductive fracture locations identified in one well aligned with corresponding features in the parallel well. An attraction of the proposed methodology is that it makes use of data that is commonly recorded during drilling, reducing the need for expensive image log tests. The information obtained from this type of analysis can be used to improve hydraulic fracture treatment designs.
The economic viability of a well drilled in an unconventional gas reservoir is largely influenced by the level of connectivity between natural fractures, stimulated fractures, and the wellbore. Engineers have the ability to control the wellbore path and, to some extent, the hydraulic fracturing process. On the other hand, natural fracture systems are outside of the engineer's control. While knowledge of the geologic conditions and stress history are helpful to estimate the characteristics of the natural fracture system in a given reservoir, the true extent of the natural fracture system in any specific location is typically unknown (e.g., near wellbore). Several well testing methods are available to the industry to identify natural fractures near the wellbore, including acoustic and resistivity image logs, but the poor-quality results of these techniques do not compensate for the expensive costs in most cases. The desire to learn as much as possible about the natural fracture systems present in tight gas plays while striving to keep drilling and completions costs to a minimum has led researchers to explore the use of drilling data as a means for natural fracture characterization.
Several reports in the literature indicate that it is possible to locate and characterize the permeability of natural fractures intersected by the drillbit during conventional, overbalanced drilling operations through the use of mud loss data (Dyke et al. 1995; Liétard et al. 1999; Lavrov and Tronvoll 2003; Huang et al. 2010). However, many wells in unconventional gas reservoirs are drilled underbalanced. For the case of underbalanced drilling, the downhole pressure condition requires that an alternate approach for natural fracture characterization be developed.
The investigation discussed within this report is essentially an exploitation study to determine whether valuable information concerning natural fracture systems that intersect horizontal wellbores in tight gas reservoirs can be discovered through the use of underbalanced drilling data. Methodologies are developed to determine highly probable conductive natural fracture "zones?? and, hence, characterize natural fracture permeability at these locations. The methods make use only of data that is commonly recorded by drilling engineers in practice during underbalanced drilling operations. Due to the high level of uncertainty inherent to this type of analysis, a practical validation analysis is identified and subsequently performed.
Hydraulic fracture simulators that include the effects of complex natural fracture systems have already been developed (e.g., Weng et al. 2011). The quantitative results obtained from the analysis proposed in this study could be directly integrated into simulators of this type, potentially reducing the uncertainty in modeling hydraulic fracture growth.
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