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To achieve optimal production from unconventional reservoirs, it is useful to determine the permeability, pore pressure, and state of stress of rock strata. Doing so will lead to properly designed treatments, realistic predictions of well performance, and a basis for normalizing reservoir contribution when evaluating completion and stimulation effectiveness.
An effective way to derive the necessary reservoir information is to conduct in-situ pressure transient tests. Since it is difficult to inject fluid into or withdraw fluid from the pore network of tight rock, diagnostic fracture injection tests (DFIT) have been employed to create an analyzable pressure decline response, as well as to derive the minimum horizontal stress via fracture closure identification.
This paper is a study of numerous DFITs conducted in unconventional reservoirs throughout the world to evaluate the reservoir and geomechanical characteristics of the pay zone and bounding intervals. Within this body of work, experiments were implemented to study the impact of testing methods on the test response and various types of analysis methods documented in the literature were implemented and compared. The paper summarizes findings and introduces tactics for planning/conducting tests and evaluating results in a variety of unconventional reservoir types.
Tinker, S.J. (Pennzoil Exploration & Production Co.) | Baycroft, P.D. (BJ Services Company) | Ellis, R.C. (Pennzoil Exploration & Production Co.) | Fitzhugh, E. (Pennzoil Exploration & Production Co.)
Mini-Frac Tests and Bottomhole Treating Pressure Analysis Improve Design and Execution of Fracture Stimulations S.J. Tinker, SPE, Pennzoil Exploration & Production Co., P.D. Baycroft, SPE, BJ Services Company, and R.C. Ellis, SPE, Pennzoil Exploration & Production Co., and E. Fitzhugh, SPE, Pennzoil Exploration & Production Co. Abstract Observations from fracture stimulations of 25 infill wells in the Grayburg / San Andres Formations at the Waddell Field in Crane County, Texas are discussed and presented. Micro-frac stress profile testing and mini-frac testing were performed to help design propped fracture treatments. Dead string pressure or bottomhole gauges were used for mini-frac and micro-frac testing. Tip-screenout methods were used to achieve desired proppant concentration in the fracture. Observations during treatments contributed to improved job design and execution. All but one well was equipped with a dead string during the frac job for observation of actual bottomhole treating pressure. Proppant transport problems detected in early treatments led to a change in fracturing fluid which allowed jobs to be pumped to completion. A wide range of leakoff characteristics was observed from well to well which made it necessary to include mini-frac testing as part of each stimulation. Large variations in measured leakoff occurred even in direct offset wells. Additionally, frac job fluid efficiency appeared higher than observed from mini-frac calibration work. The micro-frac stress test well provided a unique opportunity during the frac job to observe actual pressure in the fracture at a point away from treatment perforations. The fracture treatment communicated with the annulus through stress test perforations located above the packer. Pressure behavior observed in the fracture measured from the annulus was significantly different from pressure inside the casing at the treatment perfs. This paper presents data, observations, treatment improvements, discussion and conclusions concerning fluid selection, mini-frac and micro-frac testing, leakoff characteristics, and observations of pressure in the fracture. Introduction The E.N. Snodgrass lease is a 640-acre tract located in Waddell Field on the eastern margin of the Central Basin platform in Crane County, Texas. A map showing the field location is shown in Figure 1. The lease produces from an interval of about 300 feet in the Grayburg and Upper San Andres formations at a depth from about 3200 feet to 3500 feet. Production has been prolific and widespread from the Grayburg and San Andres dolomite formations in this area dating back to the 1920's. First production began upon discovery in the E.N. Snodgrass No. 2 in 1936. Offset development did not occur until the 1950's when 30 wells were drilled on a 20-acre pattern. Early waterflood activity began in 1967 with tour wells being converted to injection. Five successful workovers which included additional perforating and fracture treating were performed in 1993. Success of the workover program led to a 10 acre infill drilling program and full scale waterflood conversion program. Typical wells produce at rates of 40 BTFPD to 100 BTFPD. The completion and stimulation work on the 10-acre infill wells are discussed in this paper. The new 10-acre infill wells provided an opportunity to identify completion techniques that were most appropriate. A lease map with existing well locations and new infill wells is shown in Figure 2. One of the main challenges was to economically and efficiently fracture treat the 300 foot section of pay. Historically, this large interval had been fracture treated in three separate stages or by using limited entry technique with perforations throughout the entire pay interval. Mini-frac, step-rate, and micro-frac testing were all used to ensure the wells were completed effectively. Valuable and interesting observations were made during the completion program and are presented in this paper. P. 369^
New analytical pressure-transient solutions for a horizontal well intersecting multiple random discrete fractures in both an infinite and a bounded reservoir are presented. The horizontal well is assumed to penetrate multiple randomly distributed vertical fractures. The infinite reservoir Containing the horizontal well is bounded at the top and at the bottom. For the case of a bounded reservoir, all exterior boundaries are non-flow boundaries. New source functions for a random vertical fracture are first derived. The pressure-transient solutions are then obtained using the derived source functions, as well as superposition principles. A uniform flux is assumed along all fracture faces. An averaging technique is used to approximate the wellbore pressure.
The new solutions provide a theoretical basis for the analysis of the pressure transient behavior of a horizontal well intersecting multiple random discrete fractures. Different flow regimes are identified. The effects of fracture characteristics, such as fracture orientation and length, on the pressure transient behavior of a horizontal well are investigated. New evaluation techniques using the derived pressure-transient solutions can be developed to determine reservoir properties and fracture characteristics from horizontal well test data.
The PDF file of this paper is in Russian.
PKKR JSC operates a couple of dozens of fields with more than a thousand wells. Consequently, a massive wells' surveillance program is conducted which includes pressure transient testing (PTT) and analysis (PTA). It leads to different kinds of cases which, eventually, help us to understand challenging issues. Thus, in the paper we try to shed light on how these data are critical to make the right decision with the aim of getting economical benefits for the company.
Field operation performs PTT on 70-80 wells annually such as pressure buildup and deliverability on producers, pressure falloff on injectors and interference tests on observation wells. All the tests are interpreted by the engineers of the company refusing from service companies during oil price decline in 2015 and purchasing commercial software. Using the software interpreter builds Horner and log-log plots, IPR curve which give the quantitative and qualitative parameters of near the wellbore, undamaged zones and outer boundary conditions. This information coupled with geological and production data are analyzed to gather puzzles into one full picture.
As the result of PTA the company could make important decisions which brought to millions of dollars of savings and earnings. The paper demonstrates standard cases, e.g. a pressure buildup test on a well helped to decide to execute fracturing and multiple time increase in oil rate was obtained. Oppositely, on another well falloff test showed presence of fractures, hereby, planned stimulation measures were cancelled in spite of low injectivity. In addition, falloff tests give additional information on how waterflooding is effectively carried out. Another case which was under active discussion includes the issue of converting a well to water injection, so we conducted an interference test and it reveals no response on adjacent producers. Again the company avoided the wrong decision and waste of money. Moreover, interference tests played a vital role in an extraordinary case where it was unclear how the wells produced gas more than reserves. Unexpectedly, the reason was a communication between two lithologically different reservoirs via fractures. So, it dramatically changed the geological concept of the field. There are other situations when PTA results help to characterize geometry of the reservoirs, presence of faults, their transmissibility, fractures distribution which all are critical for static modeling and dynamic simulation.
PTA is a crucial part of geology and petroleum engineering. It essentially helps engineers and managers to make right decisions. Sometimes it leads to big savings or significant profit earnings due to conducting or avoiding the works. Also PTA can change the geological picture of the whole field if properly analyzed. Thus, it is a powerful tool in the hands of geologists, reservoir and production engineers.
JCPT 95-03-02 Pressure Transient Testing of the Heterogeneous Reservoirs R. PECHER, J.F. STANISLAV University of Calgary C.V. EASWARAN State University of New York, Abstract Over the years, attempts have been made to include system heterogeneities into mathematical descriptions and to determine their effect on pressure response. In this study, a general case of a system with layers of nonuniform thicknesses is considered. The mathematical description is based on assuming radial flow within individual layers and pseudosteady-state cross flow between the adjacent layers. Since the angle of the interphasing plane may assume any angle between 0 and 90 degrees, layered (uniform thicknesses) and composite systems constitute the limiting cases. The analytical solution is found in Laplace space and the numerical inversion to real space is obtained by using the Stehfest algorithm.