In this paper we demonstrate the successful identification of a new flow problem of an intersected natural fracture connected to (or near) a finite conductivity fault. In a previous paper, finite conductivity faults in connection with high permeability layers were identified in mature parts of Ghawar field(1). Four field cases are presented in this paper; a slanted well intersecting a fracture, a well close to a finite conductivity fault and a combination of the two. In the latter case, two examples of a fracture behavior at early times and a finite conductivity fault behavior at late times were identified. Because of the complex nature of these flow problems, numerical simulation along with all available data was integrated including 3D seismic to match the pressure response. Introduction This work is part of a study to characterize faults and fractures in the Ghawar field. Fractures and faults play a significant role in reservoir recovery and performance. While fractures can enhance the recovery of hydrocarbons from tight formations, the highly permeable conduits they form could also lead to a pre-mature breakthrough of water or gas. The identification and characterization of these faults/fractures network and their bhavior have become increasingly important with increased horizontal and multilateral wells drilling in this field. Thus, early identification of these geological features is a great challenge to reservoir engineers. The Ghawar field is located in the eastern part of the Arabian Peninsula. It is an anticline extending north-south of approximately 225 km in length and 35 km in width. It is divided into six operating areas. These areas are, from north to south, Fazran, Ain Dar, Shedgum, Uthmaniyah, Hawiyah and Haradh. There are ten different hydrocarbon-bearing stratigraphic horizons the Ghawar field. The two lower-most are of Devonian and early Permian clastics. The rest are carbonate deposits of late Permian and Jurasic age (Fig. 1)(2). From hundreds of field cases in heavily faulted areas, three examples were selected to illustrate some typical patterns of reservoir pressure response observed in the Ghawar field. These patterns include a slanted well intersecting a fracture (or fault) and a well close to an active finite conductivity fault(1). The third example represents a new flow system of a combined behavior of the above two examples; a well that intersects a natural fracture/fault and is connected to (or near) a finite conductivity fault (Figs 2 & 3). Well Intersecting a Natural Fracture - Case 1 This well was drilled and completed as slanted openhole oil producer to a total depth of 7270 ft across Arab-D reservoir. A flowmeter run across the entire openhole section indicates that 96% of the total oil is produced from an 8-feet interval (7080- 7088 ft). The analysis of the image log indicates the presence of a sub-seismic fracture at 7085 ft with an apparent enlargement in hole size. Openhole logs show low permeability and porosity in that zone, where the fracture is detected. Correlating all these data cofirms the presence of a fracture or a fault (Fig. 4). The derivative plot of the build up test (Fig. 5) confirms the observation made from the openhole and production logs analysis. It shows a very short wellbore storage period, followed by a long linear flow regime that dominates the rest of the test. An infinite conductivity fracture well model was used to match the pressure build up data and to obtain the reservoir parameters. Well Near a Finite Conductivity Fault - Case 2 This well was drilled and completed as a deviated openhole producer to a total depth of 7374' in July 1995, across the whole Arab-D reservoir.
The Unayzah-A in field T is an Aeolian sandstone formation with high sanding tendency requiring stimulation using the frac pack technique. Modified Isochronal Tests (MIT) and pressure buildups (PBU) were conducted on several wells in field T, to assess the reservoir and to evaluate the frac pack technique. During the analysis of the MITs and PBUs, several consistent trends were observed in the pressure derivative plots of field T wells, such as the absence of the fracture signature and the decrease of reservoir quality away from the wellbore. This consistency in the behavior of the pressure derivative was investigated in this study. Analytical and numerical well testing models, reservoir characterization, and fracturing analysis assisted in investigating the observations in PBUs. The investigation showed that to capture a fracture in the pressure derivative plot of high permeability formation, the fracture half length has to be extremely long, and there should be significant contrast between the fracture and the formation conductivities. The investigation also showed that the created fracture and the reservoir heterogeneity due to changes in geologic facies dictate the shape of the pressure derivative plot. This paper will discuss how the reservoir characterization and the fracturing analysis were used to help in analyzing the pressure transient response of frac packed wells. Two pressure buildups (PBUs) and four MITs were used in this study. This paper will also shed some light on the frac pack results, and the non-Darcy flow effect.
Injection wells are widely used in the petroleum industry to support sustained oil production. Water is injected into the reservoir for filling in the void space left by produced oil and to maintain the reservoir pressure. These injectors are frequently evaluated with fall-off tests to assess their injectivity with time. However, most fall-off tests on these wells are unexpectedly dominated by prolonged storage effects with a long unit-slope line on the log-log plot. As a result, the radial flow regime on the pressure derivative plot gets a little chance to develop within the stipulated test period. A high skin damage factor with an abnormally-high storage coefficient (e.g., 1 to 3 bbl/psi, which is one or two orders of magnitude larger than the typical values in regular wellbores) have to be introduced to the well test models to match the pressure behavior from these wells. This anomalous behavior suggests that the wellbore is connected to additional storage volume due to an induced fracture system around the well, having been intersected by a section of it. Apparently, these fractures close gradually after shutting in the well for a pressure fall-off test, manifesting as a prolonged storage effect in the data. This can be explained with the fact that most of the injectors inject water under fracturing conditions, which induce large fractures around the wellbore over time. The fracture system might play a role in taking the damage further inside the reservoir and complicate subsequent remedial work to restore well's injectivity. In this paper, we will show and discuss several fall-off tests from water injection wells in giant oil fields in the Middle East. The results of these tests, conducted over the years, are to show how the entire test duration is progressively dominated by larger wellbore storage with increasing fracture volume around the wellbore. A changing wellbore storage model (Spivey and Lee) is useful in illustrating the phenomenon of dual storage effect – one due to the wellbore, followed by the other one due to the fracture volume. The model is also used to identify the damage conditions of two regions – one just around the wellbore and the other is located slightly deeper into the reservoir.
This paper presents a successful case history of full-field reservoir characterization. It discusses real cases where transient analysis was a key tool in detecting various reservoir heterogeneities in the early phases of the field development. Pressure Transient Analysis (PTA) is well-known in the oil industry as one of the most important reservoir characterization tools for its ability to provide better identification of reservoir heterogeneities along with its well-proven capabilities in providing rational determination of reservoir parameters such as permeability thickness, skin and productivity index. Many reservoir heterogeneities such as faults, fractures, Super-K were detected along with many others. In addition the paper spots lights on many unique case histories where pressure transient analysis was able to detect an abnormality in the reservoir. The study helped enhancing the development strategy of that carbonate field following other successful studies that were previously conducted on other development fields. Introduction The field is an offshore field in the Kingdome of Saudi Arabia. It was discovered in the late 1950's and was produced sporadically. The field was initially developed with vertical producers. The structure of the field is a north-south trending asymmetrical anticline. The reservoir quality degrades towards the edges of the field. Although the initial development strategy was based on drilling vertical wells, a new development strategy that considers drilling horizontal wells was applied. The study concentrated on reservoir characterization and detection of heterogeneities along with the identification of basic reservoir properties such as flow capacity, skin and productivity index. Pressure Transient Analysis is a key tool in characterizing reservoir heterogeneities. Each reservoir feature is recognized by a characteristic shape on the pressure derivative curve. Yet, in some cases the pressure derivative curve is not conclusive by itself due to many possible reasons, one of which is short test duration. However this is can be mitigated through integration of all the available data of the well and the surrounding area. The study takes in account the history of the well from the first days of drilling until the day it was tested. It counts for very specific details that lead to extremely good reservoir characterization. Discussion Data Availability and Quality: 47 well tests data were available, of which, 90% were found to be analyzable. Figure-1 shows the distribution of the tested wells. The distribution of the tested wells was enough to provide good full-field reservoir characterization. The pressure recording of the tests were attained through mechanical gauges (AMERADA) with very low sampling frequency as shown in Figure-2. Moreover, the data does not include the drawdown period with only a single pressure point representing bottomhole flowing pressure. Finally, the build-up durations were relatively short. However, all of these limitations were overcome via integration of other available data such as drilling and completion history, formation analysis log (FAL), Production Logs (PLT), production history and geology of the area. All of the conducted tests were performed while the wells were dry, for that, all the resulted permeability figures were representative of average effective permeability for each well. The single phase flow eased the permeability calculation keeping it away from the complication of relative permeability that is usually associated with multiphase flow.
The last several years have seen an explosion in the number new fields and increments being equipped with Intelligent Field technology, which includes remote monitoring and control of wells in real-time, as well as the acquisition of high frequency pressure and rate data from permanent downhole monitoring systems (PDHMS). This paper shows how this Intelligent Field data is being leveraged to obtain full-field reservoir characterization using both analytical and numerical pressure transient analysis methods. Utilizing data from a Saudi Aramco "intelligent" field, collected over a seven-month period, the paper employs pressure transient analysis to investigate the presence of reservoir boundaries and heterogeneities, and to obtain reservoir and well properties. This work reveals the importance of Intelligent Field data, how it can add value for the pressure trainset analysis, and how it is essential in the multiwell interpretation technique to yield accurate analysis. Major heterogeneity in the reservoir of this field was successfully detected and analyzed using the numerical multiwell interpretation technique, made available by Intelligent Field technology. The results are also compared with pressure transient analysis results from wireline gauge data collected through traditional buildup tests conducted on the same wells, to illustrate the distinct advantages of using Intelligent Field data over traditional wireline test data.
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