The Grosmont formation, a carbonate reservoir in Alberta, Canada, has 400 billion bbl of bitumen resource, which is currently not commercially exploited. The carbonate reservoir is karstified by groundwater and tectonically fractured, resulting in three classes of porosity: matrix, vugs, and fractures. The viscosity of bitumen is lowered by four to six orders of magnitude when heated by steam. Since December 2010, the Saleski pilot project evaluated steam-injection-recovery processes by use of four well pairs, two each in the Grosmont C and Grosmont D units. For the first year of the pilot, two well pairs were operated with continuous injection and production similar to successful steam-assisted-gravity drainage(SAGD) projects in Alberta oil sands. Reservoir observations of steam/oil ratio (SOR) and calendar-day oil rate (CDOR) indicate recovery by gravity drainage is viable, although operating practices from conventional SAGD must be modified for the Grosmont formation. The decision to evaluate cyclic injection and production from single wells was made in early 2012, although it was recognized that cyclic operations created new challenges for the facility (which was built for SAGD operations) and artificial lift. The pilot data indicate that the drilling conditions (balanced vs. overbalanced), completions (openhole vs. slotted liner), and acid treatments of the wells have a significant impact on the individual-well performance. Injectivity into the Grosmont reservoir is high, even into a cold reservoir, because of the existing fracture system. Injection pressures stayed less than 40% of the estimated pore pressure required to lift the overburden. 4D-seismic results indicate that the injection conformance along the well axis is close to 100% and that the heated area is laterally contained around the well. Productivity is comparable to oil-sands project performance. The decline of oil rate is not only dependent on pressure but also on temperature. For cyclic operations, a CDOR of 43 m3/d (for a 450-m-long well) and an SOR of 3.4 were achieved, demonstrating that with sufficient scale, a commercial project can be established successfully. The pilot has satisfactorily derisked the Grosmont reservoir at Saleski. While cyclic operations have demonstrated economic performance, continuous injection and production similar to SAGD remains an alternative recovery strategy beyond startup in the later depletion stage. Successful future developments will advance the optimization of drilling, completion, artificial-lift, and plant capacity issues, while the reservoir itself has demonstrated its production capacity.
Improving on initial experience in the Saleski pilot with wells drilled between 2008 and 2010, a second generation well (drilled 2012) delivers economically attractive bitumen rates at efficient steam-oil ratios. This performance de-risks the reservoir and forms a solid basis for development planning. A larger scale followup project will help to optimize well spacing, multiple-well operations, management of well interference, artificial lift systems, etc. Despite comparable conditions of cyclic operation, two wells located in the same reservoir perform differently. This is related to differences in drilling conditions, well completions and stimulation methods. After pioneering horizontal drilling into the Grosmont Carbonate formation, the performance of the second generation well in Saleski is significantly improved. This paper presents the performance indicators for the pilot well and discusses the key learning steps that led to the improvement.
Edmunds, Neil Roger (Laricina Energy Ltd)
Solvent-additive processes (SAP) are a promising, but challenging technology. Perhaps the biggest challenge from an engineering point of view, is that simulators probably work some of the time, but not all of the time; and there is no information about where the line between occurs, or what the correct answer should be, after the line is crossed. Other serious problems are the many degrees of freedom in SAP process design, and the non-linear relationships between process inputs and economic results. There are too many possible designs to try randomly for even a single reservoir, and there is limited theory to interpolate or scale available experimental data.
This paper attempts to assemble some known pieces of the puzzle, and to explore how they may fit together to explain and predict SAP performance characteristics
First, some familiar PVT relationships are presented, with examples using temperature as the independent variable. This helps to clarify the choice of solvent, as a function of reservoir pressure, and also to understand the effect of the increasing solvent "dose??. It is shown that SAP will create a double front, one where the water is condensed, and a second where the solvent is absorbed by, and drains with, the oil. A vapor blanket separates the two fronts.
Secondly, simple estimates are given for the temperature distribution in the vapor blanket (i.e. solvent-active zone). Together with PVT data for the same pressure, these allow the thickness of a vapor blanket to be estimated.
Finally, SAP mass transport limits are considered, by observing that the second front essentially constitutes VAPEX. The Butler-Mokrys theory is discussed, in view of its failure to predict certain experimental results; it is argued that this results from neglect of capillary pressure effects, which in fact are dominant at the front. A purely empirical correlation by Nenniger is introduced, which can be rearranged to predict the maximum solvent speed, also as a function of temperature.
With the decrease in conventional oil and gas reserves throughout the world and an ever-increasing demand for fossil-fuel based energy and resulting high oil prices, focus has been shifting to unconventional and heavy oil and bitumen. Grosmont carbonates in Northern Alberta have been estimated to contain at least 300 billion barrels of heavy oil or bitumen. However, recovering this oil is extremely difficult because of the complexity associated with carbonate reservoirs in general, e.g., the Grosmont unit is known to possess a triple porosity system-matrix, fractures and vugs, based on core studies. The second problem is the fluid itself, which is highly viscous bitumen, immobile at reservoir conditions. To extract this bitumen from very heterogeneous carbonate rock, both heat and dilution using solvents may be needed. This paper reports the results and analysis of hot solvent experiments conducted on original Grosmont carbonate cores.
The Vapor Extraction (Vapex) process and its many hybrid variants have attracted a great deal of attention as potentially less energy intensive alternatives for exploiting heavy oil and bitumen resources. However, despite much work over the past two decades, uncertainty remains about the basic mechanisms, the scaling aspects and the most appropriate methods of numerically simulating these processes. This paper offers some insights into several of these outstanding questions. The questions are examined in the context of an extensive and well-documented set of Vapex experiments carried out by Maini and his colleagues over the past 10 years.
We have experimented with different methods of simulating these experiments using a physics-based reservoir simulator. Despite the high permeability (greater than 200 Darcys in all of the experiments), we find that capillary pressure plays a significant role in the drainage. The simulations suggest that most of the drainage takes place in the capillary transition zone along the edge of the vapor chamber, rather than in the single-phase zone ahead of it which has not yet been contacted by vapor.
It has been emphasized in the literature that the near-linear scaling of oil rate with height observed in the experiments is dramatically different from the square root of height dependence predicted by the original analytic model of Vapex. However, the experiments also show an increasing solvent fraction in the produced oil phase as height increases. When this "solvent mixing?? effect is separated out of the rates, the remaining height dependence is less than linear, though still greater than square root of height.
The relative roles of molecular diffusion and mechanical dispersion in Vapex have been widely discussed in the literature. Generally, mechanical dispersion is expected to play a larger role in these high permeability experiments (vis-à-vis the field), due to larger fluid velocities. We present a method of inferring the diffusion/dispersion present in the simulations, despite a hidden component of numerical dispersion caused by the numerical method itself. We find that the experiments are well matched with values of diffusion and dispersion in line with literature correlations, and that the contribution of mechanical dispersion is perhaps not as large relative to that of molecular diffusion as might be expected.
The paper also provides some thoughts on questions we believe are still unanswered, including mechanisms behind the height dependent mixing phenomenon and the scaling of the experimental results to the much greater heights and lower permeabilities characteristic of the field.
Many authors have published effects of Non Condensable Gas (NCG) injection during steam assisted gravity drainage (SAGD) operation, on one hand it provides an insulation blanket to the steam chamber and avoids heat loss to the over burden and improves the economics of the project, but on the other hand it can stall the steam chamber growth in the middle of high pay zone, provided the reservoir has high solution gas. All the commercial simulators predict the accumulation of the gas blanket ahead of steam front. However, field operations have proved that the NCG are produced along with bitumen and water and doesn't accumulate, but simulators are unable to predict the right amount when it comes to history matching and accurate predictions. This paper is focused on numerically findings of the gas transport mechanism in the SAGD operations. Many possible mechanisms were considered and found that most of the commercial simulators lack the function of gas production due to viscous liquid drag, which contributes a lot towards gas production especially during early years of SAGD. Solubility exclusion of the two major NCG i.e. CO2 and CH4 in both water and oil phases is another reason for under-estimating the gas production. Along with the above two mechanisms, interestingly, the constraints on the production wells in the simulators also account for a great deal of NCG production. Now instead of using a fraction of GOR, simulation engineers can include the complete GOR of the Alberta bitumen reservoirs to history match and predict the correct amount of bitumen and gas production.
SAGD is a thermal recovery process used in Alberta oil sands for over a decade as commercial process for bitumen recovery. Non-condensible gases (NCG) like methane and carbon dioxide are usually produced during SAGD operations. Methane, usually found as solution gas, comes out of the solution when the reservoir is heated. However, carbon dioxide is formed chemically from bitumen and/or minerals in the reservoir. Many efforts have been made to improve the process economics via gas and solvent injection. Prediction of the accurate NCG production is of much importance, as it influences the injection and production rates, steam chamber growth and ultimately recovery factor predicted by reservoir simulators.
The objective of this work was to investigate some possible mechanisms of NCG production, with a view to modify thermal simulators for correct prediction of NCG production and bitumen production.
The effect of solution gas on simulated SAGD performance was first studied by Gittins et al in the context of history matching an early field experiment. They concluded that it was necessary to leave gas out of the match, because including it caused the predicted rate of chamber development to be too slow. It was recognized that there was some deficiency in the existing formulations' treatment of NCG transport in steam chambers.
Thimm has pointed out that methane solubility in the abundant water phase may be a significant removal mechanism that is usually left out of simulations; but as indicated below it does not seem to be sufficient to account for the full discrepancy between field and simulations.
A number of other studies[3-6] since have confirmed the general impact of gas on SAGD simulations. Yuan et al developed experimental evidence of NCG accumulation at a steam front, as predicted (at least qualitatively) by simulation.
Edmunds presented an analysis of the effect of NCG accumulations on steam front advance. It was pointed out that the density of methane is very close to that of steam at chamber conditions, making the movement of gas at the edge of a chamber very sensitive to relatively small effects.
The Upper Devonian Grosmont Formation, located in the West Athabasca Oil Sands Deposit, contains an estimated 406 billion barrels of bitumen. The reservoir is a heavily karsted and fractured, bitumen-saturated carbonate. Initial thermal horizontal well development is currently underway in this resource. These horizontal wells have similar logistics, well construction and materials challenges to those in the McMurray Formation.
Laricina has been actively developing the Grosmont Formation. Production from the pilot began in 2011 and many lessons have been learned. The next phase of development is a 10,700 bbl/day commercial project scheduled for first steam
in 2014. The Grosmont, despite many drilling challenges such as severe lost circulation, also provides many opportunities not typically achievable in clastic oil sands developments. Carbonate rock is typically a good candidate for open-hole
completions due to its geomechanical properties. This paper will discuss the geomechanical investigation evaluating borehole stability during drilling and completion, steam injection and production operations.
Solvent SAGD hybrid processes have attracted considerable attention in recent years. The perceived benefits of solvent addition to steam in SAGD are higher oil rate, lower energy and water consumption, higher recovery by lowering residual oil saturation (Sor) and higher return on investment. Despite numerous investigations that have been published regarding different aspects of solvent SAGD processes, this hybrid process is poorly understood and the solvent effects are difficult to predict. In fact, there is no available theory to model to the transport phenomena and the role of solvent within the steam chamber. Numerical simulation studies typically model the viscosity reduction of bitumen by solvent dissolution but do not capture other plausible mechanisms that yield higher oil rate and recovery, for example, lowering of Sor or partial in-situ upgrading. Laboratory experiments at realistic reservoir conditions are needed to gain more insight into these hybrid processes.
This paper presents the results of a series of laboratory experiments for evaluation of solvent addition to SAGD. These experiments were conducted at different representative reservoir pressure in a 3-D scaled physical model. Hexane, which has shown the best performance in many studies, was co-injected as solvent with steam in these experiments. Oil rate, recovery, and steam oil ratio were compared and the hybrid solvent/SAGD process performance was evaluated at different operating conditions. Additionally post-test sand samples were extracted from the model to examine residual oil saturation in different parts of the model after each experiment. Experimental results showed improved performance of SAGD with addition of hexane, both at high and low operating pressure. However, the impact of hexane on the shape of the steam chamber and distribution of residual oil was significantly affected by operating pressure. This behavior of hexane, which appears to be related to its phase behavior, shows that solvent SAGD processes are considerably more complex than first thought.
Thermal and miscible methods are commonly used for in-situ recovery of heavy oil and bitumen. Both techniques have their own limitations and associated shortcomings, often times yielding an inefficient process. The most common thermal method is steam injection, which is highly energy intensive. Steam generation costs and water production affect the economics of the thermal technique adversely. On the other hand, miscible methods are energy effective but their economics depends on the solvent retrieval. Various combinations of these two techniques such as co- or alternate injection of steam and solvent have been proposed as a solution, but no optimum method has yet been developed.
Thermal and miscible methods can be combined by co-injecting solvent with steam or injecting solvent into a pre-heated reservoir. Current work was undertaken to study the performance of solvents at higher temperatures for heavy oil/bitumen recovery. Glass bead packs and Berea sandstone cores were used in the experiments to represent different types of pore structures, porosity and permeability. After saturating with heavy oil, the samples were exposed to the vapor of paraffinic solvents (propane and butane) at a temperature above the boiling point of the solvent, and a constant pressure of 1500 kPa. A mechanical convection oven was used to maintain constant temperature across the setup. The setup was designed in such a way that a reasonably long sample (up to 30 cm) can be tested to analyze the gravity effect. The oil recovered from each of these experiments was collected using a specifically designed collection system and analyzed for composition, viscosity and asphaltene content.
The amount of oil recovered in each case was also analyzed and the quantity and nature of asphaltene precipitated with each of the tested solvents under the prevailing temperature and pressure of the experiment was reported. Optimal conditions for each solvent type were identified for the highest ultimate recovery. It was observed that recovery decreased with increasing temperature and pressure of the system. It was also noticed that butane diluted the oil more than propane which resulted in lower asphaltene content and viscosity of oil produced with butane as a solvent.
Arseniuk, Stephen Emile (Laricina Energy Ltd)
Alberta's oil sands contain an estimated 286 billion m3 (1.8 trillion barrels) of bitumen, including more than 64.5 billion m3 (406 billion barrels) in the Grosmont Carbonate. The Grosmont at Saleski represents a significant resource, which will be developed from pilot to commercial development. The operating company plans to recover bitumen from the Grosmont using solvent-cyclic, steam-assisted gravity drainage (SC-SAGD), beginning with a pilot in the Saleski field.
The Grosmont has been noted for drilling challenges relating to lost circulation, including difficulty in cement placement. To address these challenges, lightweight, foamed, thermal cement was specifically engineered, tested, and qualified for use at the pilot. This blend addresses placement challenges and was designed to achieve the mechanical properties required for zonal isolation and well integrity throughout the life of the well.
This paper discusses the design and development of the blend using finite-element analysis (FEA) software to evaluate blend suitability, laboratory testing results to confirm basic properties, installation in the pilot wells, and results using ultrasonic cement-inspection tools.
Intro/Next Generation of Oil Sands
Before horizontal wells were possible, the Grosmont formation (located in Alberta, Canada) was piloted using cyclic-steam stimulation in vertical wells. One of the more prominent pilots was the Buffalo Creek pilot, operated from 1980 to 1986, yielding bitumen rates similar to McMurray formation pilots of similar vintage (Novak et al. 2007). At the time, neither the McMurray nor the Grosmont formations were deemed economically feasible; and through the 1980s, neither was exploited profitably. As horizontal-drilling technology became more reliable and widely used, Butler's concept of SAGD was made possible and was piloted through the 1990s, with first commercial production from the McMurray formation in 2001 (Butler 2001).
Resource estimates provided by the provincial regulator indicate that within Alberta's oil sands, the Grosmont alone accounts for 64.5 billion m3 (406 billion barrels) of bitumen (ERCB 2010). This amount is nearly three times the reserves of the surface-minable oil sands that have driven the industry for more than 40 years. The Grosmont is a heavily fractured solution brecciated dolomite with qualities such as high porosity and permeability, (Barrett et al. 2008). The operating company plans to develop the Grosmont using SC-SAGD beginning with a pilot in the Saleski Field.
The most prominent drilling challenge in the Grosmont is lost circulation that impedes cement placement. The application of foamed cement has addressed many of these challenges, and it has been used to abandon core holes and cement casing in vertical-development wells (Arseniuk et al. 2009).