Hadavand, Mostafa (University of Alberta) | Carmichael, Paul (ConocoPhillips Canada) | Dalir, Ali (ConocoPhillips Canada) | Rodriguez, Maximo (ConocoPhillips Canada) | Silva, Diogo F. S. (University of Alberta) | Deutsch, Clayton Vernon (University of Alberta)
4D seismic is one of the main sources of dynamic data for heavy-oil-reservoir monitoring and management. 4D seismic is significant because seismic attributes such as velocity and impedance depend on variations in reservoir-fluid content, temperature, and pressure distribution as a result of hydrocarbon production. Thus, the large-scale nature of fluid flow within the reservoir can be evaluated through information provided by 4D-seismic data. Such information may be described as anomalies in fluid flow that can be inferred from the unusual patterns in variations of a seismic attribute. During steam-assisted gravity drainage (SAGD), the steam-chamber propagation is fairly clear from 4D-seismic data mainly because of changes in reservoir conditions caused by steam injection and bitumen production. Anomalies in the propagation of the steam chamber reflect the quality of fluid flow within the reservoir. A practical methodology is implemented for integration of 4D seismic into SAGD reservoir characterization for the Surmont project.
Shear wave technology has had a remarkably consistent presence in the Canadian exploration community for almost 40 years. Although publications from the earliest days are sparse, Unocal and CGG were conducting some of the earliest shear-wave experiments in Canada (Omnes, 1978). Since the initiation of the CREWES Project at the University of Calgary in 1988, P-S converted waves have been acquired, processed and interpreted by many exploration companies for more than 25 years, so there is a fair amount of experience in the application of P-S converted waves to land data in the Calgary geophysical community compared to elsewhere in the world. The quality of acquisition, processing and interpretation has steadily improved over that time and the number of geophysicists who are familiar with the use of shear waves in exploration has gradually increased. Nevertheless, it would be an exaggeration to say that multicomponent exploration is widely established in Canada. Some individuals within some companies use it to gain value in their exploration effort.
Several recent examples of adding value with converted waves have primarily been applications to heavy oil in NE Alberta. Mayer et al. (2014) have demonstrated that joint prestack PP-PS inversion produced the best estimates of Pimpedance, S-impedance and density, allowing for excellent reservoir characterization of the Athabasca oil sands reservoir. The zone of interest is the unconsolidated sands of the Lower Cretaceous McMurray Formation reservoir within which the bitumen oil sands reside. Limited angles of incidence are available in the data because of a large velocity increase at the Paleozoic immediately below the McMurray. As a result, density could not be directly estimated using AVO inversion, but it was successfully estimated from the joint PP-PS inversion. Neural network analysis that used the PP and PS PSTM volumes, attributes from the PP and PS AVO analysis and attributes from the PP-PS prestack inversion as inputs further improved the resolution of the results. Figure 1 shows the result of density inversion with neural network analysis. This density inversion indicates the existence of upper and lower sands, which is also indicated by the wells, however there is a transition from bitumen to water within the sands that is not indicated by density.
During steam assisted gravity drainage (SAGD) processing of an oil sands reservoir, two marked phase changes occur. First, bitumen itself changes from quasi-solid to liquid when heated. Second, a steam chamber develops as the liquid bitumen is displaced with steam during production. Therefore, steam chamber development is a direct indication of production performance. A good understanding of the developed chamber extent and future chamber growth is essential for a successful SAGD project. In this paper, we present an integrated approach to quantitatively predict chamber geometry, fluid saturation and temperature. We define the chamber in three classes, based on phase changes: (1) developed steam chamber zone (high steam saturation and temperature, indicating completed production); (2) mixed fluids zone (low steam saturation and high temperature, indicating near-future production); and, (3) transition zone between quasi-solid to liquid (warmed reservoir with future potential),
The above workflow was applied to the Nexen Long Lake SAGD project using the 2002 baseline and 2014 monitor seismic data. The developed steam chamber (class 1) was predicted for 2014. The 2014 monitor data were also used to predict the future extent of the developed chamber for the year 2016 (using classes 1 and 2). The predicted chambers for both 2014 and 2016 were found to correlate well with production data.
Presentation Date: Thursday, September 28, 2017
Start Time: 11:25 AM
Presentation Type: ORAL
Time-lapse seismic (4D) analysis of a heavy oil reservoir undergoing steam injection is a critical step in the evaluation of production efficiency and wellbore conformance. Through the identification of changes in seismic character, steam chamber development can be mapped as three-dimensional bodies to monitor active steam fronts and identify unheated portions of the reservoir.
The Nexen Energy ULC (“Nexen”) time-lapse project (the “Project”) is an innovative approach to 4D monitoring, where small, individual arrays of permanently installed geophones are constructed on a fit-for purpose basis, unique to each drainage area. This enhancement allows for quick, cost effective acquisition, and short processing and interpretation timelines.
Time-lapse monitoring of steam-assisted gravity drainage (SAGD) bitumen reservoirs is an essential step to maximize production over the life of a project. The identification of unheated portions of the reservoir helps to improve bitumen recovery. Unheated resources can be targeted and drilled in subsequent reservoir exploitation to help maximize recovery. Furthermore, mapping 4D anomalies along a horizontal in the early stages of production can lead to better conformance, improved cumulative steam to oil ratio (CSOR; the cumulative ratio of steam injected to oil produced over the life of a well), increased bitumen production, and ultimately enhanced cost efficiency and profit margins.
Effective and accurate seismic monitoring of reservoir changes is dependent on a number of factors. The simplicity of shooting and processing repeat surveys may be compromised by changes in acquisition parameters, differences in processing techniques, source and receiver repeatability issues, and evolving geological knowledge of the reservoir (Kelly, 2013). Minimizing changes through uniform acquisition and processing of time-lapse surveys is a critical step in achieving accurate time-lapse interpretations (Li et al., 2001).
This paper will be an overview of the project, including improved acquisition design, analysis of 4D results, and an integrated, multidisciplinary approach to interpretation.
Lerat, Olivier O. (IFP) | Adjemian, Florence (IFP) | Baroni, Axelle (IFP) | Etienne, G. (IFP) | Renard, Gerard (IFP) | Bathellier, Eric (CGGVeritas) | Forgues, Eric (CGGVeritas) | Aubin, Francois (CGGVeritas) | Euzen, Tristan (IFP Technologies)
This paper presents an integrated workflow for the interpretation of 4D seismic data to monitor steam chamber growth during the steam-assisted gravity drainage recovery process (SAGD). Superimposed on reservoir heterogeneities of geological origin, many factors interact during thermal production of heavy oil and bitumen reservoirs, which complicate the interpretation of 4D seismic data: changes in oil viscosity, fluid saturations, pore pressure, and so on.
The workflow is based on the generation of a geological model inspired by a real field case of the McMurray formation in the Athabasca region. The approach consists of three steps: the construction of an initial static model, the simulation of thermal production of heavy oil with two coupled fluid-flow and geomechanical models and the production of synthetic seismic maps at different stages of steam injection.
The distribution of geological facies is simulated on a fine grid using a geostatistical approach, which honours all available well data. The reservoir's geomechanical and elastic properties are characterized by logs and literature at an initial stage before the start of production. Production scenarios are run to obtain pore pressure, temperature, steam and oil saturations on a detailed reservoir grid around a well pair at several stages of production. Direct coupling with a geomechanical model produces volumetric strain and mean effective stress maps as additional properties. These physical parameters are used to compute new seismic velocities and density for each stage of production according to Hertz and Gassmann formulas. Reflectivity is then computed, and a new synthetic seismic image of the reservoir is generated for each stage of production.
The impacts of heterogeneities, production conditions and reservoir properties are evaluated for several simulation scenarios from the beginning of steam injection to 3 years of production. Results show that short-term seismic monitoring can help in anticipating early changes in steam injection strategy. In return, long-term periods allow the behaviour of the steam chamber to be monitored laterally and in the upper part of the reservoir. This study demonstrates the added value of 4D seismic data in the context of steam-assisted heavy oil production.
The Steam-Assisted Gravity Drainage (SAGD) process has been successfully implemented to produce ultra-viscous bitumen from the Athabasca oil sands in the province of Alberta. In the Hangingstone area, 15 pairs of SAGD wells had been drilled in the reservoir by 2006, each a maximum of 30 m in thickness and approximately 300 m in depth. The production reached an average of 8,000 BOPD in recent years. The reservoir is geologically characterized as a stacked incised valley with fills in fluvial to upper-estuarine channels. Thin mudstone layers and abrupt changes in facies caused by sedimentary deposits present complexities and difficulties for SAGD implementation.
A 3D seismic survey was conducted in 2002 to obtain a clear view of geology that was fully utilized for planning additional wells. In order to evaluate SAGD efficiency and performance, a time-lapse 3D seismic survey was carried out in 2006. In this paper, P-wave velocity (Vp) maps, transformed from the seismic travel-time maps, were interpreted with a new methodology for evaluating the areal extent of the steam chamber zone created by the SAGD process. In the previous experimental study of seismic velocity measurements with oil sands cores, Vp was found to steeply drop with an increase in temperature and to gently decrease with an increase in pore-pressure.
Based on the experimental results, a petrophysical model was formulated to express Vp as a function of temperature, pressure and water saturation. The high-pressure and high-temperature zone of the SAGD process should generate differences between the first (2002) and second (2006) Vp maps from which we can estimate the area of the reduced bitumen viscosity with a temperature increase. As effects of pressure are probably more areally extensive than effects of temperature, these two effects on the Vp maps need to be segregated. As a new method, a scaling factor for the Vp reduction was first estimated to adjust the laboratory-scale and field-scale. We then calculated a distribution of Vp reduction corresponding to steam chamber conditions in order to decouple composite effects of temperature and pressure, based on the petrophysical model. Distinguishing the high-temperature and high pore-pressure zone from the low-temperature and high pore-pressure zone, we could determine a steam chamber distribution.
The bitumen volume in the steam chamber zone was estimated and compared with the actual production. The methodology, interpretation procedures and results obtained are presented in detail.
Toe-to-Heel-Air-Injection (THAI™) is an in-situ combustion process that is used for the recovery of bitumen and heavy oil. It combines a horizontal production well with a vertical air injection well placed at the toe. Compressional and shear velocities for heavy oil are extremely sensitive to temperature. As the oil is heated by the combustion process the velocity decreases causing distinct time-delay anomalies on timelapse (4D) seismic. These time-delay anomalies indicate that the combustion front is moving from the toes of the wells, where the air injectors are located, towards the heels. Downhole thermocouples are used to measure the temperature and hence provide control/calibration points.
This paper presents a new interpretation methodology of 4D seismic data to determine the steam-chamber distribution generated by the SAGD operation in the Athabasca oil sands. Thin mudstone layers and abrupt changes in facies present difficulties upon SAGD implementation. 3D seismic surveys were conducted in 2002 to aid in understanding of the facies distribution, and in 2006 to evaluate SAGD performance.
Two methods are demonstrated to estimate the steam-chamber areal distribution. The first approach is based on comparison of seismic traveltime maps from the two 3D surveys, and the second method uses the interval P-wave velocity (Vp) from the top to the bottom of the reservoir transformed from the seismic traveltime. We formulated a petrophysical model that expresses Vp as a function of temperature, pressure, and water saturation based on the previous experimental measurements of seismic velocity with oil-sands cores. Scaling factors for Vp reduction were first estimated to adjust the laboratory scale to field scale and distributions of Vp reduction and traveltime changes corresponding to the steam-chamber conditions were calculated.
Vp and traveltime maps that reflect the high pressure and high temperature zones generated by the SAGD process were obtained and distributions of Vp reduction and traveltime were calculated with the petrophysical model in order to decouple composite effects of temperature and pressure. Effects of pressure were assumed to be areally more extensive than temperature effects. By distinguishing high temperature and high pore-pressure zones from low temperature and high pore-pressure zones, the steam-chamber distribution was determined. The steam-chamber distribution obtained by the traveltime approach did not show a very good agreement with the well production performance, while the Vp approach presented consistent results. The bitumen volume in the steam-chamber zone estimated by the new approach was calculated, and compared with the actual bitumen production.
The methodology demonstrated here can be applied to other 4D seismic data at fields under thermal recovery processes.