Production Analysis of Long-Term Linear Flow in Tight Gas Reservoirs: Case Histories
- Jorge A. Arevalo-Villagran (Texas A & M University) | Robert A. Wattenbarger (Texas A & M University) | Fernando Samaniego-Verduzco (UNAM) | Tai T. Pham (El Paso Production Company)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 30 September-3 October, New Orleans, Louisiana
- Publication Date
- Document Type
- Conference Paper
- 2001. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.5.8 History Matching, 5.8.6 Naturally Fractured Reservoir, 2.2.2 Perforating, 4.1.5 Processing Equipment, 1.8 Formation Damage, 5.1.5 Geologic Modeling, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.6.4 Drillstem/Well Testing, 5.8.1 Tight Gas, 4.6 Natural Gas, 5.4.2 Gas Injection Methods, 5.5 Reservoir Simulation
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Actual field data show linear flow in a large number of tight gas wells. Sometimes this linear flow is transient for years. Linear flow is normally associated with hydraulic fractures. Short-term linear flow production analysis may characterize fracture treatments, but long-term linear flow production may be controlled in some cases by the reservoir geometry; in others, it is controlled by the natural occurring reservoir properties. In this paper, long-term linear flow caused by both the presence of natural parallel fractures and vertical flow in a high permeability streak is investigated. An analytical matrix- parallel fracture model for a single-phase flow is presented. A systematic procedure to analyze linear flow in tight gas wells is described. Application of this methodology to production analysis from several tight gas wells and validation of the results by using numerical simulation are also shown.
Linear flow is characterized by behavior during transient flow. This is sometimes associated with hydraulically fractured wells with linear flow perpendicular to the fracture. At the end of linear flow, the pressure responses (for a constant rate solution) of these wells flatten as flow enters from outside the fracture tips1,2. However, this paper refers to observed well behavior in which the pressure response becomes steeper at the end of linear flow, indicating an outer boundary effect. For these wells, there appears to be only linear flow during transient and outer boundary dominated flow. Actual field data shows long-term linear flow for years in a large number of wells3-12 because of the extremely low permeability.
A half unit slope of a log-log plot of ? m(p)/qg vs. t for either constant gas rate production, qg, or constant bottomhole flowing pressure, pwf, indicates linear flow. Long-term linear behavior has been reported in tight gas wells which did not have particularly large fracture treatments7,9,11. The reason for linear flow may not be known for a particular well. But several papers discuss physical scenarios which may cause linear flow5,7,11,13,14, including the occurrence of natural fractures and massive tight formations which could be expected to have layers of higher permeability.
In this paper, we show how parallel natural fractures lead to permeability anisotropy that causes linear flow and how a high permeability streak causes vertical linear flow. Afterwards, we outline a straightforward and stepwise methodology for analyzing wells with long-term linear flow.
Finally, we show four field examples where transient and boundary dominated flow periods are detected and analyzed.
Physical scenarios that cause linear flow
Parallel natural fracturing.
Long-term linear flow in tight gas wells may develop because of large permeability anisotropy ratios. Anisotropic permeability in porous medium has been examined in several papers15-25 and books26-31. Permeability anisotropy may occur for a variety of reasons. One of the most important reasons is the presence of parallel natural fracturing.
Tectonic stresses determine the direction of both hydraulic fracture and natural fractures. These natural fractures would tend to be parallel to the hydraulic fracture plane and would cause linear flow even if the fracture length were limited. However, if the tectonic stresses have changed since the formation of the natural fracturing, the hydraulic fracture could have a different orientation from the natural fractures15.
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