Characterization of Tight Reservoirs
- W.J. Lee (Texas A&M U.) | C.W. Hopkins (S.A. Holditch & Assocs.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- November 1994
- Document Type
- Journal Paper
- 956 - 964
- 1994. Society of Petroleum Engineers
- 5.1 Reservoir Characterisation, 5.3.2 Multiphase Flow, 5.4.2 Gas Injection Methods, 5.6.9 Production Forecasting, 5.6.4 Drillstem/Well Testing, 5.6.1 Open hole/cased hole log analysis, 4.1.2 Separation and Treating, 5.8.2 Shale Gas, 4.1.6 Compressors, Engines and Turbines, 5.5 Reservoir Simulation, 2.5.2 Fracturing Materials (Fluids, Proppant), 3 Production and Well Operations, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.6.9 Coring, Fishing, 1.6 Drilling Operations, 2.4.3 Sand/Solids Control, 5.8.6 Naturally Fractured Reservoir, 2.5.1 Fracture design and containment, 5.5.8 History Matching, 5.8.1 Tight Gas
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Distinguished Author Series
This paper gives methods to characterize tight gas reservoirs in sufficientdetail to allow an engineer to make accurate long-range production forecasts.These forecasts are the basis for sound engineering and business decisions.Because of the complexity and variability of tight gas reservoirs, we canpresent only general procedures for developing reservoir descriptions.Accordingly, we illustrate a reservoir characterization method with threeexamples of successful tight gas reservoir studies. The procedures in theseexamples can be modified as needed for other specific formations or areas.
Production rates from many tight reservoirs are marginal, but thesereservoirs account for a large percentage of the long-term gas supply. Becauseof the marginal economics, efficiency is the key to drilling and producingthese tight reservoirs. To optimize production, we must have a goodunderstanding of the reservoir, but often the economics cannot supportcollecting the data necessary to describe the reservoir properly. A reservoirengineering study for tight reservoirs requires us to balance data collectioncosts with the level of detail necessary to describe the reservoir accurately.One must determine what level of reservoir characterization is needed tooptimize production from tight reservoirs efficiently. Unfortunately, becauseof the diverse nature of tight reservoirs, there is no single answer. Thequestion must be answered on a case-by-case basis.
Reservoir studies of tight reservoirs are performed to meet many differentobjectives. Because tight wells require hydraulic fracturing, fracturetreatment optimization studies are quite common. A reservoir study is sometimesperformed in conjunction with a detailed geologic study to help identify keywell characteristics or field trends to be used as exploration tools and topredict reserves. A reservoir study can identify infill-well potential and thepotential for increased productivity and reserves as the result of theinstallation of compression or liquid lift equipment. Finally, reservoirstudies can resolve conflicting data or determine why some wells are notproducing as expected.
Unfortunately, analysis of tight reservoirs is one of the most difficultproblems facing a reservoir engineer. Many tight formations are extremelycomplex, producing from multiple layers with permeabilities that often areenhanced by natural fracturing. Unfortunately, low productivity and marginaleconomics often prevent expenditures of money and time to collect the dataneeded for a detailed reservoir study. Because the permeability of theseformations is low, many standard formation evaluation techniques do not provideadequate results. Standard log-based correlations for permeability or otherproductivity indicators often fail in tight reservoirs, so correlations must bedeveloped on an area-specific basis. Many tight shale reservoirs haveproductive gross intervals exceeding 300 ft, making it difficult to determinewhere the gas is produced, thus complicating completion decisions. Even intight gas sands made up of interbedded sands and shales, layering can have apronounced effect on well production. Natural fractures often occur in thesetight formations, making wells that appear similar on logs perform quitedifferently.
When we do not describe the reservoir in sufficient detail, the productionforecasts we generate are frequently wrong. Unless we can predict postfracturewell performance accurately, we cannot optimize the fracturing process. Soundbusiness decisions regarding compressor installation, infill drilling, orremediation treatments are not possible. Unfortunately, for layered reservoirs,oversimplified reservoir descriptions frequently result in an overestimatedwell productivity.
Fig. 1 shows predicted 20-year performance for a Devonian shale well forthree different reservoir descriptions: "lumped" one-layer, 3-layer,and 10-layer reservoirs. All three predictions are based on the same gas inplace and the same total permeability-thickness product, kh. Note that thelumped one-layer model overpredicts the gas recovery by a factor of two. Thefour-layer model prediction is closer to actual but is still high by about 17%.Any business decisions based on the single-layer prediction would be seriouslyin error.
Development of tight gas reservoirs has been increasing substantially overthe last decade. Because of this trend, the Gas Research Inst. (GRI) and theU.S. DOE have been funding detailed research in tight gas sands and shalesthroughout the U.S. This research has led to significant advances in hydraulicfracturing and a better understanding of the complexity of the tightreservoirs.
The importance of describing a layered reservoir has been discussed in theliterature for several decades. Much of this discussion has centered onpressure-transient analysis of layered reservoirs (with and without crossflow)and descriptions of the nonideal buildup test pressure responses often observedin the field. Lefkovits et al. presented analytical solutions for flow inlayered reservoirs and identified several characteristic features of reservoirswith discrete, noncommunicating layers. Other investigators presented numeroussolutions describing pressure and flow rate that include the effects ofinterlayer crossflow, stimulation, or unsteady- (transient) orpseudo-steady-state (boundary-dominated) flow. Comprehensive analyticalreservoir models have been developed specifically to model the pressure or flowrate response from layered reservoirs.
Although much theory has been presented in the literature, case studiesdocumenting layered reservoir analyses are not as common. Much has beenpresented on fracture treatment optimization in tight reservoirs, but generallythe impact of layering is not discussed. The majority of layered reservoiranalyses involve history matching data by use of reservoir simulators, althoughthe recent availability of comprehensive analytical models should make layeredanalyses easier and more cost-effective.
Even with sophisticated computer programs for analysis of layeredreservoirs, this analysis is still not straightforward. Cost-effective datacollection methods for describing layered reservoirs are not developed easilybecause of the diverse nature of tight reservoirs and widely varying productionvolumes.
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