Figure 3. This image of a steaming operation in a heavy oil reservoir depicts the heightened microseismic activity occurring at the extremity of the pad. This could be an indication of a possible breach into the inter-pad buffer zone.

Microseismic (passive seismic) monitoring is a burgeoning technology in the petroleum industry. Commonly employed to visualize hydraulic fracture stimulations, microseismic monitoring can also be used for reservoir characterization, providing E&P companies with real-time information on the dynamic state of their assets. Producers can use this geophysical technique to unlock the hidden characteristics of a reservoir, enabling better decision-making, risk avoidance and, ultimately, optimized asset production.

One of the contributing factors to what makes microseismic monitoring such an ideal solution is the ease in which it can be integrated into the decision-making process that producing companies use to manage their E&P assets. Upstream producers entrench these processes into their management systems to ensure decisions and actions are aligned with obtaining maximum value from producing assets. Microseismic monitoring solutions can enhance these decision-making processes, creating a more efficient system for asset management and reservoir optimization.

This article will describe how reservoir characterization through microseismic monitoring adds value at each stage of a producer’s asset management system. The methodology described herein can be applied to virtually any asset management model that uses a similar decision-making procedure.

Asset management

Figure 1 illustrates a generic asset management process. Although most asset management systems are unique to the individual company, the overall process procedure is usually the same. Data from the field are acquired and processed into a useable format. The useable data are then analyzed and entered into a producer’s specific asset model for interpretation. The information gleaned from these models becomes the basis for making and executing operational decisions designed to increase the lifecycle value of the reservoir.

The continuous process employs various components and activities to streamline management decisions towards achieving asset optimization. Microseismic monitoring contributes to the process by adding value to each of the four key phases.

Phase 1: Acquiring and processing data

The first step in the asset optimization process is to obtain the necessary raw data that will provide insight into the specific characteristics that influence the reservoirs behavior. A monitoring system is designed to detect, locate and capture the microseismic events that occur within the reservoir. Before a monitoring system can be installed, it is important to understand the underlying geological characteristics of the reservoir that will affect microseismicity. Information such as dipole sonic logs, depth, temperature, pressure, lithologic configuration and well layouts are all used to custom-design a monitoring system. A typical system is comprised of sensor arrays, usually deployed in an adjacent observation well; data acquisition units; a central gathering station computer; and finally a satellite link that transmits the seismic and engineering data (pressure, temperature, tilt) to the remote offices for further processing.

Once the relevant data reaches the office, a team of geophysicists and seismologists begin processing the raw seismic data, identifying the useable seismic events that can be interpreted to model the reservoir behavior.

Phase 2: Interpretation and modeling

In the next phase of the process, geophysicists analyze the data, using proprietary software and visualization tools to create images of the microseismic events. These images are used to characterize the reservoir and provide understanding of what is occurring subsurface. Microseismicity identifies the strains associated with reservoir compaction, which can be investigated to ensure reservoir caprock integrity. Similarly, microseismic activity in the overburden or rock mass deformations can indicate regions of active deformation that may be susceptible to well casing failures. Visualization of microseismic events will also reveal the existence of any undetected fault lines or fracture networks that may lie within the reservoir formation (Figure 2). All these characteristics are input into the producing companies’ reservoir model to visualize current operations and look at different responses to production.

For example, in a reservoir using cyclic steam stimulation, the microseismic images will depict where in the reservoir the injected steam is going. The fracturing associated with the steam flow can be mapped to indicate if the steam is mobilizing the oil as planned or escaping along pre-existing fault lines, leaving the target area untouched. Sometimes producers have constructed models (for injection) based on pre-existing reservoir data. Microseismic data will help to validate these existing models and can provide some directionality in terms of re-optimizing based on newly discovered information. Integrating microseismics with a producer’s asset model delivers a clear picture of the underlying reservoir characteristics. Best of all, permanent microseismic monitoring provides a continual inflow of real-time information which can be used to ascertain the long-term effect these characteristics are having on production operations.

Phase 3: Formulating and evaluating decisions

With microseismic data integrated into a reservoir model, decision-makers gain timely access to accurate information upon which to generate and evaluate potential courses of action. For instance, in the cyclic steam stimulation example, the asset team can now begin to construct different production options in response to the steam compliance. Figure 3 illustrates an example where a large amount of microseismic activity has been detected near the extremity of a well pad. In this situation, engineers may choose to cease or adjust steaming operations to avoid breaching the buffer zone between adjacent pads. Fracture networks can be a key factor for introducing steam into less permeable areas of the reservoir, so steam migration routes can be delineated and incorporated into future well placement design and drilling operations. Similarly, if steam is found to be escaping down a pre-existing fault structure, the asset team may decide to relocate the injection well or even decide to seal the fracture. The microseismic data collected in the field becomes the foundation upon which these key decisions are formed.

Phase 4: Implementation and reservoir optimization

The final stage of the value management system is focused on executing the action plans that will drive reservoir optimization. Control systems are established to measure the effectiveness of new procedures and to help lock in gains. In a case where a reservoir is experiencing heightened microseismic activity in the overburden, engineers may choose to establish a set of criteria for identifying potential well casing failures and caprock breaches. In addition, classification schemes for casing failures could be developed and automatic response strategies (such as e-mail or cell phone alerts) could be designed to alert the necessary personnel if microseismic events exceed pre-designated thresholds. Steam flow efficiency can be monitored by measuring improvements in steam-to-oil ratios and by continually reviewing updated microseismic images of steam development within the reservoir to ensure compliance. As the cycle repeats itself, producers will also be able to integrate engineering data with active seismic (4-D) surveys and use this “full picture” of information as a historical baseline to monitor the change in characteristics over the lifetime of the asset.

As the rate of discovery for new reservoir assets continues to decline, it is becoming more important for producers to optimize existing assets to increase their lifecycle value. Microseismic monitoring, along with traditional pressure and temperature monitoring, is a powerful reservoir characterization tool which delivers valuable real-time information on activities that directly impact production. By integrating this technology into an E&P asset management process, producers can increase the efficiency of their operations and reduce potential risks, creating a sustainable model for optimized asset production.