recovery) phase. For the very reason, the notion of reservoir characterization often times means different thing to geologists, geophysicists and reservoir engineers. This is primarily due to the fact that their primary focus is different phases of life of the field.
1.4.1 Exploration Stage/Development Stage
The pre-development (exploration) phase requires delineation of the reservoir limits and assessment of its economic feasibility. Development stage requires somewhat more accurate assessment of the reservoir extent, better appraisal of the economic viability of the reservoir and placement of new wells for further delineation of the reservoir. More detailed depth imaging using advanced geophysical methods, borehole geophysics applications as well as fault seal analysis, and better understanding of reservoir compartmentalization are some of the objectives of reservoir characterization at this stage. At this stage it is necessary to determine the reservoir drive mechanism and the size and strength of the aquifer. At the development phase, reservoir continuity rather than reservoir delineation becomes the focus. The focus in this phase is to identify the structural extension or truncation beyond well control in order to minimize the drilling of dry holes. While the pre-development and development phase require considerable attention to reservoir characterization. Geophysicists construct an initial model by correlating lithology, porosity, net pay thickness and other properties at the well location using seismic attributes from surface 3D seismic, VSP and synthetic seismograms. The initial model is then updated by computing seismic attributes between the wells in order to predict reservoir properties in 3D. Seismic attributes like acoustic impedance, variations in amplitudes, frequencies, interval velocities and instantaneous phase are applied in the computation.
1.4.2 Primary Production Stage
As the primary production of the reservoir begins, the goal is to position wells at optimal locations that would maximize hydrocarbon recovery. During secondary recovery and then enhanced recovery process, the engineer’s objective is to maximize the volume of hydrocarbon contacted by injected fluids. This is to achieve maximum volumetric sweep efficiency for fluid production. To minimize cost and risk, engineers attempt to predict reservoir performance—for both planning and evaluation of hydrocarbon recovery projects. Reservoir description in terms of reservoir architecture, flow paths, and fluid-flow parameters are the key to reservoir engineering. Accurate prediction of reservoir production performance is predicated primarily on how well the reservoir heterogeneities are understood and have been modeled and applied for fluid-flow simulation. This stage requires integration of reservoir characterization models with reservoir simulation, history matching for production optimization. Reservoir management process conducts reservoir related studies and applies the results from fluid flow modeling in defining, updating and optimizing a development plan for producing the reservoir and forecast the production profile. This phase also involves optimization and management of reservoir performance evaluation, surveillance of fluid flow and changes in the reservoir that result in changes in the original distribution of physical properties. The optimization criteria can change during the life cycle of a producing reservoir. Managing the reservoir depletion is a dynamic process and the reservoir engineers constantly react and adapt to the changes as they evolve. Dynamic characterization is a representation of the fluid flow in a static reservoir model and needs to be validated with reservoir performance data.
1.4.3 Secondary/Tertiary Production Stage
When the reservoir ages, intervention to increase production through reservoir stimulation and enhanced oil (or gas) recovery becomes necessary. Here, the main objective shifts to plan the production and injection, develop an optimum secondary/tertiary recovery mechanism and plan for infill drilling well locations to maximize recovery rate and deliver oil and gas at the planned rate and to extend the economic producing life of the reservoir as much as possible. The finite resources that are available are applied to plan and optimize the economic recovery of oil and gas from the reservoir. Petroleum engineers are responsible for planning and executing the development and of petroleum reserves. They seek to maximize petroleum recovery from the reservoirs. Effective use of reservoir characterization and reservoir model updating play an important role in this process. This is accomplished by incorporating physical property measurements of the reservoir rock and fluid properties that describes the reservoir architecture and its initial fluid distribution to make an optimum production and reservoir stimulation plan. 4D seismic and/or micro-seismic monitoring plays a role in assessing the effectiveness of well different well stimulation approaches (for example EOR, artificial lift or hydraulic fracturing), giving rise to the importance of “dynamic reservoir characterization” at this stage.
Engineers need to monitor the reservoir state of pressure, temperature and fluid distribution during the producing life of a reservoir. This information could 1) identify situations within the reservoir which may potentially impact oil and gas recovery, and 2) locate problems that can cause undesirable leakage or entry into wellbore. If these situations are not corrected in a timely manner, irreversible damage might occur to the reservoir affecting the ultimate oil and gas recovery. Inter-well monitoring of production and injection processes using geophysical techniques also allow improvement of field development plans and optimize reservoir management. Field scale monitoring of reservoir drainage patterns would improve the recovery factor.
1.5 Dynamic Reservoir Characterization (DRC)
Both in the primary and post-primary production phases, we need to have an updated characterization of the reservoir. We will refer to this as DRC. DRC can play a key role in production optimization and monitoring of effectiveness of the EOR operation or hydraulic fracturing. It can help surveillance of fluid flow which is an essential part of reservoir management process. Likewise, changes in the reservoir pressure distribution is also helpful to make important reservoir management decisions. Among many data sets that are used to monitor fluid, pressure and other reservoir properties is 4D seismic and Microearthquake data. For some examples, see Kosco [7], Maity [9] and Maleki [10].
1.5.1 4D Seismic for DRC
Changes in the reservoir fluids or pressure distribution can be imaged by 4D. This is based on the impact of fluid saturation and reservoir pressure on the changes in compressional and shear wave velocities of seismic waves. A simplistic idea is to directly subtract two 3D seismic volumes of acquired in say successive years. This is often referred to as 4D seismic, with the fourth dimension being the time lapse between the two 3D seismic surveys. An example of time lapse seismic is in Figure 1.6. As the reservoir is produced, the distribution of the fluid properties changes with time but the reservoir rock frame properties remain constant during the producing life of the reservoir. Therefore, by repeating the seismic measurements i.e., acquiring time lapse seismic data and computing the differences in the seismic attributes between the measurements the changes in the distribution of reservoir fluid properties between wells could often be detected.
Figure 1.6 Time-lapse seismic response changes caused by different positions of oil-water contact (OWC) in Gullfaks field Tarbert reservoir. (a) Oil-water contact level before production and (b) after production for 10 years. Zones with changes in seismic impedance are circled. http://accessscience.com.
The changes in time lapse seismic data are due to the acoustic impedance variation (in this case, mostly the compressional velocity change), caused by the reservoir production or other changes (such as water or CO2 injection in the EOR process). Acoustic impedance is the product of velocity (V) and density (ρ), for time lapse seismic amplitudes are influenced by the incompressibility