Peripheral water flooding has been the preferred pressure maintenance tool for many gulf carbonate reservoirs over the past 30 years. Due to uneven sweep and pressure distribution, this technique has given way to pattern floods in several gulf fields. As these new floods are established, it is important to understand the water saturation between wells to properly manage the sweep and recovery. In 2007, ADCO initiated water injection (WI) and WAG pilots to test the recovery strategy. The pilot employs advanced geophysical and modeling tools to measure formation properties at the wells and between wells; this paper discusses the WI pilot. Among the novel techniques applied is the crosswell electromagnetic method, which measures the interwell resistivity distribution between observation wells at the pilots. Interwell resistivity data can be used to infer the water saturation distribution because of the sharply different electrical resistivity between injected water and oil bearing reservoir rock. By allowing an evaluation of the water distribution long before the injected fronts reach producers or observers, a better and more rapid understanding of the pilot arises from the crosswell electromagnetic technique. In this paper, we briefly describe the pilot design, describe the detailed geological model and show results from the initial set of baseline and time lapse EM data sets from the water injection pilot. The images highlight the influence of background geological constraints on the flow. Introduction Applying peripheral water flooding for pressure maintenance was commenced after few years of the discovery of field A, a giant complex carbonate reservoir in the middle-east. Although this strategy has been successful, there is evidence of an uneven sweep due to reservoir complexity. These complications have led to the introduction of pattern-based flooding technology and the establishment of the water injection (WI) and WAG pilots in the underswept lower units of the reservoir. The benefits of pattern flooding are more efficient and faster recovery. The potential drawbacks are greater costs and higher local pressures which could induce uneven flows. Detailed pattern flood modeling helped develop an optimum strategy for maximizing reserves and production, especially in the lower two oil bearing units of the reservoir (Ref. 1). Consequently, WI pilot has been implemented in the lower units of the reservoir. A detailed multi-year and multi measurement monitoring plan has been established to determine the pilot performance which includes deep reading technologies like electromagnetic surveys. The main objectives of the WI pilot project are:determine sweep efficiency in the target reservoir units,qualitatively assess the impact of injected fluid fluxes vertically across low permeability sub-units within the reservoir, anddetermine pressure support due to pattern injection. The pilot will also address the issues of uneven sweep, bypassed oil, and residual oil saturation. The acquired field data will be used to calibrate the simulation model for production, injection, saturation and pressure data in order to design as an optimum field development scheme for the lower reservoir units in the southern part of the field. (Ref 2)
Time-lapse cross-well electromagnetic (EM) surveys are used to monitor two types of fluid injection (Water Injection and Water Alternating Gas) in a giant field in the Middle East. Cross-well EM data will help optimize sweep efficiency, identify bypassed pay, and predict fluid-related issues such as water breakthrough by providing an image of the resistivity distribution between boreholes in time lapse. This paper explores the influence of a high quality background geologic model in constraining the interwell results and providing a higher resolution image of the ongoing flooding processes. The classic EM inversion process determines a coarse (3 to 5 m resolution) resistivity distribution from a basic initial static reservoir model built from logs. This study refines the model by adding variable resolutions to encompass the small-scale heterogeneities common to carbonate reservoirs. Incorporating geological data derived from seismic attributes, core descriptions, and detailed log analyses into the static model helps optimize the EM inversion and increases the resolution of the resulting inverted model. Introduction A few years after the discovery of a giant complex carbonate reservoir in the Middle East, Giant Field A (Fig. 1), peripheral water flooding was successfully initiated to maintain pressure. Recently, it appears that reservoir complexity has led to uneven sweep. ADCO is currently testing pattern-based flooding technologies to improve sweep efficiencies at two pilot studies—the water injection (WI) and water alternating gas (WAG) pilots—to monitor the under-swept lower units of one of the main reservoirs in this field, which are not being swept with the peripheral flood. Although pattern flooding leads to more efficient and faster recovery, some potential drawbacks include greater costs and higher local pressures, which could induce uneven flows. Detailed pattern flood modeling helped develop an optimum strategy for maximizing reserves and production, especially in the lower two oil bearing units of the reservoir (Bhatti et al. 2006). Consequently, water injection has been implemented for these lower units of the reservoir. A detailed multi-year and multi-measurement monitoring plan has been established to determine the performance of this pilot study, including deep reading technologies like cross-well electromagnetic surveys. The WI pilot was designed to determine sweep efficiency in the targeted reservoir units while assessing the impact of injected fluids on low permeability subunits and monitoring pressure support due to pattern injection. Uneven sweep, bypassed oil, and residual oil saturation are secondary considerations of the WI pilot study. Production, injection, saturation, and pressure data will be used to calibrate the simulation model. The ultimate goal is to design an optimum field development scheme for the lower reservoir units in the southern part of the field (Bhatti et al. 2007). Assessing pilot performance and fine-tuning the model's predictive capabilities requires proper surveillance, planning, and timely data gathering. To efficiently meet the pilot objectives while acquiring high-quality inter-well data, traditional methods were enhanced by the addition of more advanced (deep reading) methods. Selected well-based monitoring methods include well logs and pressure and flow data. Evaluation of a number of advanced geophysical methods led to the selection of the crosswell EM method for interwell saturation monitoring (Bhatti et al. 2007). This paper shows the results of the first time lapse in the WI pilot, where decrease in resistivity due to 4 months of water injection in the lowermost units of the reservoir have been identified and interpreted with respect to the geological understanding of the pilot area.
Modeling of water saturation during simulation of this middle-eastern super giant carbonate reservoir is quite challenging due to its geological complexities. This reservoir is multilayered, thick and very heterogeneous with permeabilities ranging from 0.01 mD to several Darcies. Consequently, the log-derived water saturations as functions of height above the free water level span a wide range in values. For poorer quality reservoir, the range is larger even when the S w (z) values are subdivided into their respective rock type groups. A database of experimental SCAL data acquired over the last 50 years was utilized to prepare saturation functions to initialize the water saturation in the simulation model.A workflow for generating internally consistent sets of saturation functions for simulation input from capillary pressure and relative permeability data (Kralik, J.G., et. al., 2010) was applied to the SCAL database. The data were reviewed to confirm that they were of acceptable quality for use in saturation function development and organized by rock types. For each rock type, saturation endpoints were first established by curve fitting the appropriate capillary pressure data to a consistent constitutive equation that contains the saturation endpoint as a regression parameter. The resulting primary drainage water-oil P c model was checked against the available log S w (z) data, especially those at the top of the formation. Once the saturation endpoints had been established, relative permeability saturation functions were derived, starting with the bounding water-oil primary drainage curves, followed by the primary imbibition k row -k rw curves. Three-phase gas-oil primary drainage k rog -k rg bounding curves were derived in an analogous manner. Because the imbibition k row -k rw tests correspond to unique imbibition scanning curves, they were scaled to the bounding primary drainage k ro curve to ensure consistency. This process was applied to each group of capillary pressure and relative permeability data. Data gaps were filled with suitable analog SCAL data taken from the rock type groups that were the most similar. The process used above provided for self-consistent primary drainage and imbibition functions.The prepared saturation functions (J-function and relative permeability) were input into the simulation model. The quality of the water saturation match as predicted by the simulation model was checked against log data for 120 wells. A good initialization of the model was achieved with some exceptions. The relatively poor quality unit at the bottom of the reservoir showed a mismatch, especially in the stylolitic intervals. The main mismatch was due to the larger range in permeability values (i.e., two cycles on logarithmic scale) for the same rock type. One of the rock types was further subdivided into distinct rock types for the porous and the stylolitic units. This modification greatly improved the water saturation match of the model. Model S w was further improved in the northern part of the reservoir b...
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractMajor contribution to oil production has been mostly from the highly prolific upper Southern units in a giant complex carbonate reservoir in the middle-east region. The water front advanced much faster in highly permeable upper units than the lower units and has created uneven water sweep in southern part of the field. Reservoir management program has been implemented to reduce production from wells that are located near the water finger area to achieve an even water flood advance. However, the current situation has raised some issues concerning the optimization of recoverable reserves in lower units due to water slumping from upper to lower units, uneven areal and vertical sweep, and uncertainty in the effectiveness of dense intervals within the lower subunits. But opportunity still exists to improve the recovery from these units.
In the paper, we will briefly review the pilot design and demonstrate the utility of applying the EM imaging to the pilot. We will also show the benefit of the optimized casing material on the resolution of the crosswell EM resistivity images and describe the methods employed for monitoring the fluid flow and show preliminary results of the modeling process. This crosswell EM technique which has been successfully employed and proven in other geographical areas is being implemented first time in UAE. The EMI technology is being deployed in southern part of a complex carbonate reservoir in the middle-east where an uneven flood front advance has been observed in different reservoir units. It has been observed that water front has advanced much faster in the highly permeable upper reservoir units as compared to lower reservoir units. In order to understand the horizontal and vertical fluid flow behavior, an inverted 5-spot water injection pilot pattern is being implemented. The pilot will address the issues of the uneven sweep efficiency, bypassed oil and effectiveness of stylolites across different units. The pilot results and observed data will be used in the simulation to design an optimum development scheme for the lower reservoir units in the southern part of the field. The current dynamic simulations predicted that the injected water will reach producers after 7 to 10 years. However, the decision on field developments have to be taken early enough to avoid the slumping of water from upper to lower units and loss of reserves in the lower units. Early imaging of the injected water from the injection well into the reservoir is paramount in assessing the success of the pilot and future field development issues. It is anticipated that this tomographic Cross-well Electro-magnetic (EM) resistivity technique will provide sufficient imaging information to track the water flood movement between wells. The most favorable conditions to acquire reliable formation resistivity distribution information, EMI require at least one kilometer distance or separation between wells. Prior to the field deployment, simulations were run to confirm the applicability of the technique and define the parameters for the survey with objectives; 1) to check the sensitivity of EM technique to the reservoir conditions and injected fluids, and 2) to carry out actual EM tool simulation and check the quality of tool response. The study concluded; 1) cross-well EM resistivity technique is well suited for tracking the water front in the current reservoir conditions, 2) the injected fluids create enough resistivity contrast to be easily picked up by the technique, and 3) the flood front progress can be captured by conducting the surveys in a time-lapse mode. As part of this project, lab tests were conducted to choose a material that would limit the attenuation at high frequency as much as possible at source and receiver locations. Introduction Peripheral water flooding as pressure maintenance method was commenced within few years of the discovery of the giant complex carbonate reservoir in the middle-east region. Although initially quite successful, the field has experienced uneven distribution of water flood front, vertical and lateral sweep due to reservoir complexity. Detailed simulation modeling was performed to delineate optimum strategy for maximizing reserves in the lower two oil bearing units of reservoir thereby controlling the process at a more local level (Ref. 1). The benefits of this process are more efficient and faster recovery. The potential drawbacks are greater costs and higher local pressures which could induce uneven flows.
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