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L-S Sands, Mata Field, Eastern Venezuela Basin: Multidisciplinary Approach and its Role in Hydraulic Unit Characterization*
By
José Renato C. Peron1, Jorge Arguello1, and Denis Marchal1
Search and Discovery Article #40152 (2005)
Posted May 3, 2005
*Adapted from extended abstract, prepared by the authors for presentation at AAPG International Conference & Exhibition, Cancun, Mexico, October 24-27, 2004.
1Gerencia Reservorio, Petrobras Energia Venezuela, Torre Lamaletto, Av. Venezuela, El Rosal, Caracas, 1060, Venezuela, phone: 00-58-212-9577401, fax: 00-58-212-9577403 ([email protected])
Abstract
In mature fields that present multiple, stacked oil sands, the trinomial “production / injection / pressure” in some cases may not behave as a coherent data set. In such cases, the first approach is to search for possible communication between sands. The understanding of the different communication processes is a key to optimization of field exploitation. Commonly, analysis of the communication is not evident, mainly when (1) the communication presents severe restrictions or (2) when it is induced, partly or totally, by the exploitation (mechanical induced communication in wells or breaking of fault seal due to differential pressures).
In Mata field, a detailed geological analysis shows the possible communication within the L sand package and within the S sand package either by sand coalescence (erosion processes) or/and by the juxtaposition of the sands across faults.
Nonexistent at the beginning of the field production, the communication between the L package and S package became evident after having produced about 20% of OOIP. Systematic material balance studies and analysis of the production history and of the volumes of currently mapped gas cap strongly suggest induced communication between the originally isolated sand packages, offering coherence to the entire history. Although not totally understood, the induced communication process is analyzed, and all hypotheses considered.
For L and S sand packages, communication is consistent with a field history wherein sands are analyzed under grouped sand criteria (Hydraulic Units). As a result of this work the commingled exploitation of these sands was authorized and is being implemented in Mata field.
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IntroductionAlthough historically production in Mata field was ruled by the concept that every sand is hydraulically isolated from the others, analysis of the production, injection, and pressure of all sands of L and S packages shows evidences of communication between the reservoirs. Pressure measurements in the last few years indicate common pressures for the sands at a same datum, even though they presented distinct recovery factors. Besides the anomalous behavior of the production and pressure of the field, material balances show evidences of mass flux within and/or between the sands (of the same units) or between the L and S sand packages. In order to understand the communication between the sands, several geological factors have been considered. Structural juxtaposition of porous sections from the different reservoirs across faults and mechanical failures in wells (with more than 50 years of life) are proposed as the main ways of communication. Exhaustive material balances and production analysis were made in order to understand the communication, estimate the amount of mass exchanged between the sands, and evaluate its impact on the potential of the whole field.
GeologyField Location and Regional Geology Mata Field is located in the Greater Oficina trend in the southern flank of Maturin sub-basin, representing the oriental part of the Eastern Venezuelan Basin (Figure 1). This foreland basin was formed starting from the oblique convergence, in a dextral movement, between the Caribbean and the South American plates (Lugo and Mann, 1995). The Greater Oficina trend is characterized by extensional tectonics with regional dips trending N to NE.
Geologic Characterization: Specific Objectives As part of reservoir characterization, emphasis was focused on the necessity of understanding the communication mechanisms between distinct sands. To complete the current geological model, a 3D geological model was build in order to: 1. Determine the role of the geological factors in the communication between the reservoirs (stratigraphic characteristics, structural features, etc.). 2. Characterize in detail the sand-sand communication zones across the faults:
Stratigraphic Characterization Sequence stratigraphy, mainly based on well logs and core analysis, was analyzed from basin scale to a sand-reservoir scale. The system tracts were defined and classified (lowstand, transgressive and highstand). (Picarelli et al., 2001). The L-S sands are associated with incised valley fills in channelized deposits in a lowstand systems tract. The incised valleys, generated by lowering of sea level, can produce contact within sands whenever the sealing horizon is thin. Such a “kink” in sand communication has been identified in the L and S sands (Figure 2).
The structure is characterized by a system of normal faults. Fault displacements result in the juxtaposition of distinct reservoirs across faults (Badley et al., 1991). Based on the 3D geologic model, 3D juxtaposition maps were made (“Allan Maps”) over the entire fault planes to analyze the spatial distribution of the fault displacements and of sand porosity. Assuming that the fault itself do not act as a seal, the potential communication zones (leakage zones) were identified by the simultaneous analysis of (i) the juxtaposition between sequences and within the sequences, for each individual sand, across the fault planes; and of (ii) the spatial (geostatistical) distribution of the porosity on the same fault plane (Figure 3). These zones of potential communication possess lengthened form, and they are distributed in an irregular way on the fault planes. This geologic study demonstrates that, besides the erosive events that can constitute communication mechanisms between sands, the displacements caused by the secondary faults may also induce communications between the sands in the field.
ProductionIn Mata field there are nine main reservoirs with a total OOIP of 200 MMSTB and cumulative production of 48 MMSTBO. Figure 4 shows the initial pressure, the current pressure, and the recovery factor of the L-S sand package. It shows that with different recovery factors the actual pressure is almost the same for all sands, even though some sands have secondary recovery and others do not. Due to the communication between the L-S sand package, the traditional material balance technique was not able to fit the field history; therefore, the material balance equation was adjusted to identify the influx/outflux (source term: M) necessary to adjust the history-- M = F - N * ( Eo + m*Eg + Efw)
Another technique implemented was to identify “the total gas available to be produced” (Figures 5 and 6), as given here below: The solution gas produced with the produced oil + the original gas cap + the injected gas + the gas that comes out of solution (due to the depressurization of the reservoir)) - the gas trapped by the critical saturation. The idea is that the total available gas minus the produced gas is equal to the actual free gas. A comparison of this volume with the volume of the actual gas cap, volumetrically calculated with the actual gas/oil contact, possibly indicates gas flux into or out of the sand package.
Description of Material BalanceIn order to apply the material balance, the L-S sand package was divided in four groups, L12, L34, S12, and S34. Figures 5 and 6 illustrate the influx/outflux calculated, for 2 sand packages (L12 and L34), in order for there to be a fit of the sand group history and of the produced gas with the available gas for production.
Sand Package L12In the beginning there was a loss of mass from the L12 sand package; between 1962 and 1975 the sand received mass and stayed more or less stable after that (Figure 5). At the end of 2002 the sand had lost around 3 MMBbl (reservoir condition). The actual produced gas is 40 MMMSCF, and the actual available gas is 53 MMMSCF (Figure 5); therefore, there ought to be around 13 MMMSCF of free gas in the sand. The actual mapped gas cap size shows 18 MMMSCF of free gas, with a difference of -5 MMMSCF.
Through all the production time, the sand package has received an influx (Figure 6), having received, to date, around 21 MMBbl (reservoir condition). The total gas available to be produced is 21 MMMSCF and the sand produced 40 MMMSCF (Figure 6). In other words, the sand produced 19 MMMSCF more than it would be possible if it were an isolated sand. This excess of gas certainly came from others sands. Currently, there is no identified gas cap in the sand. The two others sand packages were analyzed the same way. The source term (M) of all four sand packages is shown in Figure 7. A table of the actual gas cap volume, the produced gas, and the available gas to be produced is shown as Figure 8.
The assumption that the L-S sand package is a hydraulic unit implies that it is a closed system, and consequently the entire package should not lose or gain mass. Therefore, if inside the package one sand loses mass, it is due to the fact that another one is gaining it. Based on this concept, the summation of all influx should be zero. The graphics of the influxes (with the summation) and the pressure history are shown in Figure 7. In conclusion, at the beginning the L sands were in communication and the S sands were not. After 1977 all the sands became in communication, as seen in both plots (Figure 7), where all the pressures are similar, at a common datum, and the summation of the influx in all sands tends to be zero. Although there is a huge error in the gas volumes calculated by the gas balance when analyzed sand by sand, it is clear that the total error is small. This means that there was a strong gas movement from the lower sand package S to the upper sand package L. This fact leads to a hypothesis that the communication occurred at the top of the structure.
ConsolidationIn the beginning only the L sands behaved as a hydraulic unit; the geological model easily explains this fact. In 1977 almost all sands in the L-S sand package started to behave as a hydraulic unit. The geological model cannot explain this fact, and although some models have been studied, there is no proved reason for this. Based on the theory that the communication occurred at the top of the structure, the two most reasonable hypotheses are the breaking of the seal of the main fault or mechanical failures in wells.
In Mata field a multidisciplinary approach was the key to understand the communication between sands historically exploited as individuals reservoirs. This approach was the support for achieving legal authorization to exploit all L-S sand packages as a hydraulic unit.
AcknowledgmentWe thank Petrobras Energía de Venezuela for allowing us to publish the results of this work.
ReferencesBadley, M.E., Freeman, B., Roberts, A.M., Thatcher, J.S., Walsh, J., Watterson, J. and Yielding, G., 1991, Fault Interpretation during seismic interpretation and reservoir evaluation, in The integration of geology, geophysics, petrophysics and petroleum engineering in reservoir delineation, description, and management: AAPG Special Volume (Proceedings First Archie Conference), p. 224-241. Lugo, J., and Mann, P., 1995, Jurassic-Eocene tectonic evolution of Maracaibo Basin, Venezuela, in Petroleum basins of South America: AAPG Memoir 62, p. 699-725. Picarelli, A. Holder, B., Argüello, J., Perón, R., Nieves, I., and Rojas, J., 2001, High-resolution sequence stratigraphy and reservoir characterization applied to mature fields: Example from Eastern Venezuela Basin: SPE 69602 (March, 2001). |
