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Cap rock integrity in Alberta's Oil Sands has gained increasing industry prominence over the years. A competent cap rock seal is a key mandate to subsurface containment assurance in thermal operations such as Steam Assisted Gravity Drainage (SAGD). Containment loss incidents in the past decade present substantial insights into regulating thermal development prospects as well as defining and benchmarking the industry practices in Alberta. Cap rock characterization and its response to high pressure and temperature in SAGD greatly influences the reservoir management strategy adopted by the operators. Constraints on the Maximum Operating Pressure (MOP) and safety factors are generally premised on tensile or shear cap rock failure probabilities.
This work integrates and analyzes key industry data from subsurface disciplines of geology, geophysics, geomechanics and reservoir engineering in characterizing regional Clearwater and Wabiskaw shale cap rocks in the Athabasca basin. A comprehensive analysis was conducted on sixteen (16) commercial oil sands projects and incident reports. Applications, reports and Supplemental Information Requests (SIRs) submitted to the Alberta Energy Regulator's (AER) published data and relevant literature was consulted to generate regional interpretations of the cap rock properties and industry approaches. A regional database of key properties including In-situ stresses, horizontal stress anisotropies, pore pressure gradients, and rock mechanical properties was compiled. In addition, regional failure modeling practices including numerical modeling assumptions, coupling, initial and boundary conditions and failure criteria are studied. Finally, common reservoir and cap rock monitoring techniques are explored.
Major conclusions from this study include regional interpretations of various risk factors affecting cap rock integrity in Oil Sands. Inferences from pooled industry data is used to generate a holistic interpretation of the Wabiskaw and Clearwater cap rocks. Intrinsic risk factors embedded in commonly practiced cap rock evaluation techniques, modeling and surveillance techniques in SAGD operations are identified alongside containment assurance programs commonly adopted by industry stakeholders. A summary of findings is provided at the end of this study for Operators to consider advancing their view on subsurface containment risk management.
Multicomponent joint inversion is an important technique for reservoir prediction using PP and PS seismic data. The addition of PS data is helpful to solve the problem of multiplicity and increase the precision of reservoir prediction. Based on 3D multicomponent seismic data of M area in Canada, the logging response characteristics of the reservoir are analyzed and the sensitive parameters are optimized. The PP and PS joint inversion, characteristic curve inversion and lithofacies probability simulation are integrated to increase the precision of reservoir prediction gradually. The application results show that, due to the reservoir prediction based on joint inversion, the top and bottom interface of oil sands reservoir and the distribution of interbed are described in detail. And important geophysical prospecting results are provided for oil sands development in this area.
Presentation Date: Monday, October 15, 2018
Start Time: 1:50:00 PM
Location: 213A (Anaheim Convention Center)
Presentation Type: Oral
ABSTRACT: The main goal of this research was to investigate the risk of caprock failure due to the SAGDOX process, a hybrid steam and in-situ combustion recovery process for oil sands. A temperature dependency extension to the linear and non-linear constitutive models was developed and implemented in the GEOSIM software. The analysis has shown that there is no increased risk of caprock failure for SAGDOX process compared to SAGD. The study has shown that the overlying Wabiskaw formation experiences shear failure during both SAGD and SAGDOX due to its low initial cohesion, friction angle and proximity to pressure and temperature front, although the failure was mainly driven by pressure propagation. However, Clearwater shale above Wabiskaw can still provide proper zonal isolation to the steam/combustion chamber under SAGDOX operating conditions. Uncertainty in the analysis is due mainly to the sparse nature of geomechanical properties data for the oil sand reservoir and the caprock formations, especially at temperatures over 200 C.
1.1 The SAGDOX process
Nexen Energy ULC (Nexen) has been evaluating SAGDOX - a post SAGD oxidation process (Kerr, 2012; Jonasson and Kerr 2013) - to improve the recovery and project economics of its Long Lake SAGD operation. SAGDOX is meant to be used after several years of SAGD operations when the bitumen between two SAGD well-pairs is mobile. In SAGDOX process (applied to a row of parallel well pairs) oxygen is coinjected with steam in every other SAGD injector well and starts an oxidation process by reacting with residual oil around the injection well. At this point the SAGD production well below the oxygen-steam injector is shut in and steam along with oxygen and combustion gasses fill the steam chamber voidage and push hot bitumen towards the neighbouring SAGD well-pair. The neighbour injection well is also shut-in and could be converted to a producer if need be. Various other well arrangements have been considered including those with vertical injection wells and infill horizontal production wells. Since oxygen is co-injected with steam, very high oxidation temperature of a pure combustion process are not generated as steam carries a large portion of the heat of combustion away from reaction front and temperatures are thereby moderated. Nonetheless, temperatures in the range of 400-600 deg C are expected in the oil sand zone. The high temperature combustion front where the oxidation reactions are active moves away from the oxygen injection wells as the residual oil left behind after steam displacement is consumed. The high temperature reaction zone has a tendency to move upward towards the cap rock under the influence of gravitational forces.
Polymer flooding is an Enhanced Oil Recovery process usually employed in tertiary mode, after a waterflood. However, in heavy oil reservoirs the process is also commonly employed either in primary or secondary conditions because waterflooding is not considered to be economic. For new projects the question often arises as to whether it is better to start by injecting water or to go straight to polymer injection. Injecting water first provides a baseline for comparison with the polymer flood and in some cases allows an earlier start-up of the project due to the construction and commissioning of polymer mixing facilities. On the other hand, the risk is that water channeling could occur and could be irreversible, thus reducing the potential recovery during polymer injection.
Some authors have reported laboratory studies comparing the different methods but so far there has been no field results published that could be used for validation or guidance. Although several polymer flood projects in heavy oil have been reported in the past few years they are still relatively few and no detailed field results have been reported to date on that issue.
The largest polymer flood in heavy oil is currently being implemented in the Pelican Lake field in Canada, with several hundreds of horizontal wells injecting polymer. All the methods - primary, secondary and tertiary - have been tested and as a result the field provides a large database that allows to compare recovery and other parameters for each method.
Although such a comparison is mostly focused on a single field and is thus inherently biased, it should provide useful guidelines for companies looking to start new pilots or field projects. Relevant literature on this topic will also be reviewed to present a complete review of the issue.
This paper presents a numerical assessment for the Maximum Operating Pressure (MOP) of a Steam Assisted Gravity Drainage (SAGD) project considering the effect of the Natural Fractures (NFs) and intrinsic anisotropy of the cap shale. Current numerical and heuristic assessments usually ignore the effect of the intrinsic and structural anisotropy of the cap shale in the caprock integrity studies.
A coupled Hydro-Thermo-Mechanical (HTM) model was developed to assess the MOP. The coupled model employed a novel constitutive model which was developed to investigate the effect of NFs and intrinsic anisotropy in the cap shale. The coupled model was validated against surface heave measurements, and later utilized in a sensitivity study to assess the MOP for cases with different number of NF sets, fracture density and fracture dip angle.
Results indicate that the MOP is highly sensitive to the fracture density and dip angle. According to the results, vertical fractures have minor effect on the MOP while oblique fractures with the dip angle between 25° to 65° significantly affect the MOP. Neglecting the NFs can lead to significant overestimation of the MOP. This highlights the necessity to include the NFs in the caprock integrity assessments.
The numerical model presented in this paper considers the intrinsic anisotropy and the presence of NFs in the cap shale, while existing mathematical tools for caprock integrity studies have not incorporated the intrinsic and structural anisotropy. Ignoring the anisotropy in caprock can potentially cause a considerable overestimation of the MOP.
The Pelican Lake field in northern Alberta (Canada) is home to the first successful commercial application of polymer flooding in higher viscosity oils (i.e. greater than 1,000 cp), which has opened up new opportunities for the development of heavy oil resources.
The field produces from the Wabiskaw "A" reservoir which has thin pay (2 to 6 meters) and exhibits a significant viscosity gradient across the field, with oil viscosities as low as 600 cp in the existing waterflood and polymer flood area to over 200,000 cp in the current undeveloped "immobile" area. This unique geological feature limits the application of chemical injection to the less viscous areas of the field and calls for different methods for the heavier accumulations.
As a first step to develop alternate technologies capable of recovering oil from heavier areas of the field not ideal for polymer flooding, a hot water injection pilot was designed and implemented in June 2011. The hot water injection scheme was applied to a transition area where dead oil viscosity ranges from 3,000 cp to approximately 15,000 cp. It consists of one horizontal producer supported by two horizontal hot water injectors, with an injector-producer distance of 50 meters for both injectors, and 3 vertical observation wells equipped to monitor pressure and temperature between one injector and the producer.
The pilot was operated in three phases. The first phase consisted of 6 months of primary production period to obtain a baseline of the pilot performance prior to hot water injection. The second phase began in June 2011 and consisted of hot water injection through the edge injectors. The third phase was started in March 2012 and consists of hot water edge injection accompanied by hot water circulation in the production well as a means to stimulate oil production. One of the features of this stage is the use of an insulated coil tubing, which continuously delivers hot water to the toe of the producer and allows continuous stimulation and uninterrupted oil production.
This paper describes the mechanical components of the pilot and discusses the results obtained with an emphasis on the hot water circulation stage which has proven to be very effective. Oil production increased from approximately 6 m3/d during the flood stage, to more than 25 m3/d during the hot water circulation stage and has held relatively steady for more than 2 years.
The data captured has been reconciled with analytical and reservoir simulation models, and results suggest that the technology could help unlock some of the heavier oil accumulations in the field.
This paper investigates the effect of anisotropic behavior of caprock shales on the caprock failure pressure in SAGD projects. Shales and mudstones exhibit strong anisotropy at the micro and macro scales. However, the anisotropic behavior has been neglected in the existing published works on this subject.
In this research project, a coupled hydro-thermo-mechanical model was developed for the assessment of caprock integrity in thermal operations. A transversely isotropic constitutive model in the elastic range was combined with an anisotropic failure criterion to capture the intrinsic anisotropy of the cap shale. The coupled tool was validated against field data and employed in a study to determine the effect of shale anisotropic behavior on the pressure associated with caprock breach.
Results display the effect of shale anisotropy on caprock response in terms of deformations, stresses and failure pressure. The assumption of isotropic shale behavior in caprock integrity assessment for a case study resulted in the overestimation of the failure pressure by about 10%.
Existing numerical models for evaluating the integrity of caprocks during thermal operations employ isotropic constitutive laws. These models are believed to be deficient in capturing strongly anisotropic response of shales and mudstones. The research described in this paper incorporated elasto-plastic shale anisotropy in the caprock failure analysis model for the first time. The paper demonstrates the importance of capturing shale anisotropy in the accurate prediction of caprock breach pressure in SAGD projects.
Temperature falloff logs are routinely performed on a number of operating wells at Statoil's Leismer field to monitor well integrity. So far, all wells have demonstrated normal temperatures in formations overlying the steam chamber, less than steam temperatures, and with falloffs according to typical heat conduction. However, one injector well did not falloff as expected after a 24 hour shut-in; instead showed temperatures exceeding 200 C in formations above the steam chamber. An investigation was launched and a multidisciplinary team evaluated possible causes for the high temperatures. A review of 4D seismic data showed that a neighbouring steam chamber was intersecting the wellbore. This led to the hypothesis that the high temperatures were the result of wellbore heating from the neighbouring steam chamber, and a followup, extended temperature falloff log confirmed the hypothesis. The extended logging period confirmed that temperatures outside casing did falloff, albeit at a slower rate than other wells, due to the heating effect of the neighbouring steam chamber. The temperature log measurements indicate that heat from an intersecting steam chamber can cause a counterintuitive segregation of steam and water in the wellbore, with condensed liquid water held above steam in the upper section of the well. This case study highlights strategies for interpreting downhole temperature anomalies using temperature logs and 4D seismic, and also the benefits of a well integrity investigation with strong cross-discipline collaboration.
The Pelican Lake heavy-oil field in northern Alberta (Canada) has had a remarkable history since its discovery in the early 1970s. Initial production by use of vertical wells was poor because of the thin (less than 5 m) reservoir formation and high oil viscosity (800–80,000-plus cp). The field began to reach its full potential with the introduction of horizontal drilling and was one of the first fields worldwide to be developed with horizontal wells. However, with primary recovery at less than 10% and 6.4 billion bbl of oil in place (OIP), the prize for enhanced oil recovery (EOR) is large. Initially, polymer flooding had not been considered as a viable EOR technology for Pelican Lake because of the high viscosity of the oil, until the idea came of combining it with horizontal wells.A first - unsuccessful - pilot was implemented in 1997, but the lessons drawn from that failure were learned and a second pilot was met with success in 2006. The response to polymer injection in this pilot was excellent, with oil rate increasing from 43 BOPD to more than 700 BOPD and remaining high for more than 6 years; the water cut has generally remained at less than 60%. Incremental recovery over primary production is variable but can reach as high as 25% of oil originally in place (OOIP) in places. This paper presents the history of the field and then focuses on the polymer-flooding aspects. It describes the preparation and results of the two polymer-flood pilots, as well as the extension of the flood to the rest of the field (currently in progress). Polymer flooding has generally been applied in light- or medium- gravity oil, and even currently, standard industry-screening criteria limit its use to viscosities up to 150 cp only. Pelican Lake is the first successful application of polymer flooding in much higher-viscosity oil (more than 1,200 cp), and as such, it opens a new avenue for the development of heavy-oil resources that are not accessible by thermal methods.