Practical Aspects of Reserves Determinations for Shale Gas
- Richard F. Strickland (The Strickland Group, Inc.) | Dwayne Christopher Purvis (The Strickland Group, Inc.) | Thomas Alwin Blasingame (Texas A&M University)
- Document ID
- Society of Petroleum Engineers
- North American Unconventional Gas Conference and Exhibition, 14-16 June, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 3 Production and Well Operations, 5.1 Reservoir Characterisation, 5.7.2 Recovery Factors, 6.1 HSSE & Social Responsibility Management, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 5.3.1 Flow in Porous Media, 5.2 Reservoir Fluid Dynamics, 5.7.5 Economic Evaluations, 1.10 Drilling Equipment, 5.1.2 Faults and Fracture Characterisation, 1.2.1 Wellbore integrity, 1.8 Formation Damage, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.8.2 Shale Gas, 4.6 Natural Gas, 5.8.9 HP/HT reservoirs, 1.2.3 Rock properties, 5.1.5 Geologic Modeling, 5.7 Reserves Evaluation, 4.3.4 Scale, 5.6.1 Open hole/cased hole log analysis, 5.6.9 Production Forecasting, 5.5.2 Core Analysis, 2.5.4 Multistage Fracturing, 5.8.1 Tight Gas, 1.6 Drilling Operations, 5.5.8 History Matching
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Shale gas currently provides 20% of domestic supply, is targeted by half of the gas-directed drilling rigs, and represents the large majority of domestic resources. However, modern shale plays, their development strategies and their engineering analysis are young by comparison to those of conventional reservoirs. Uncertainty in shale gas reserves has significant implications at both the micro and macro levels.
Conventional reservoir engineering tools must be viewed as potentially inadequate (or even inappropriate) for the evaluation of shale gas performance primarily because of the extremely low aggregate permeability of these systems, but also because of other unique aspects of the systems. Reservoir modeling (simulation) has an important role as an assessment and prediction tool; however, the character of the reservoir (induced and enhanced natural fractures) must be considered, as well as the geological and fluid characteristics. Rate-transient analysis (modern decline analysis) techniques are also more rigorous and have been expanded and adapted to fit the uniqueness of shale gas production. Application of each method for shale gas is discussed, including methods and limitations. These two techniques more closely represent the physics of shale gas production, but their implementation is often prohibitive.
By way of necessity, much engineering evaluation is performed using Arps decline curve analysis. This technique is argued by some to be inappropriate due to a lack of theoretical support and demonstrated tendency to over-estimate reserves in tight gas systems. Given the limitations, practical methods exist to reduce error associated with its use. A newer decline method, power-law exponential, is also investigated.
Gas shales currently supply 20% of gas production in the United States, and the majority of gas resources in the United States. It has been the target of major capital expenses in recent years and probably represents the cause of the current gas supply glut. Expansion of gas shale plays to other parts of the world is gaining momentum.
Despite the massive capital investments made in recent years, the science of shale gas analysis and forecasting is relatively young. Horizontal wells with large, multi-stage fracture treatments became the standard protocol for gas shales in 2005, a mere six years ago, and only a few hundred wells were drilled in that year.
Though around twenty thousand horizontal shale wells have been drilled to date, the longest actual production history available is about six years. To further complicate matters, the usually frenetic and sometimes frothy nature of shale development creates an urgent need for accurate predictions of recovery very early in the life of a play.
Unfortunately, geologic and reservoir engineering principles for analysis of gas shales have lagged the science of hydraulic fracturing. In the early days of the Barnett Shale expansion, it was widely believed that regional variations in the quality of the formation was relatively minor. Production mechanisms were poorly understood, as was petrophysical analysis. It was argued, and is to certain extent still, that completion technology was the primary determinant of production and thus economic success.
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