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| The coring tool provides operators with samples of freshly cut core, from thousands of feet below Earth’s surface, while capturing the samples’ associated gases and fluids. |
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The ever-increasing global demand for oil and gas has driven more operators to target unconventional shale plays. This increase in activity is also driving demands for new technologies that maximize efficiencies, in assessing the economics of these resource plays, while simultaneously maintaining the highest safety standards.
One key area of technological growth lies in the evaluation of potential hydrocarbons present in the reservoir, in the hopes of future economic production. For operators, understanding the true potential of any drilling program is crucial.
Key indicators of a play’s potential performance can be determined by the amount of hydrocarbons in a targeted formation. If the volume is significant enough, the investment is, typically, worthwhile, despite potential challenges. Accurately quantifying resource values in place, for oil and gas reservoirs, requires a multidisciplinary effort involving rock physics, gas and fluid geochemistry, crushed rock analysis, and the integration of subsurface pressure and temperature data.
To address these challenges, Corpro, a division of ALS Oil & Gas, designed a coring tool that provides operators with samples of freshly cut core, from thousands of feet below the earth’s surface, while capturing the samples’ associated gases and fluids. This evolving technology takes the guesswork out of reservoir characterization and, in turn, helps operators make critical, fact-based decisions that boost future production efforts.
Accurately calculating the level of hydrocarbons, and the composition of a shale gas or oil reservoir, requires maintaining the integrity of core samples. Using traditional coring methods, in-situ gases and fluids often leak out of the sample, and the loss of these elements can significantly reduce a comprehensive, accurate understanding of what is trapped below the earth’s surface.
To complete a successful core analysis program, strict procedures must be adhered to, to ensure that the core is cut properly, recovered on surface, and transported to the laboratory. Common mistakes in any of these three areas often result in poor sample quality, and inaccurate or missing data, which is paramount to the decision-making process.
Corpro’s specialized tool, QuickCapture, encapsulates a pseudo pressure core sample from a reservoir in a sealed system, which is designed to capture virtually all in-situ gases and fluids, at safe working pressures, inside the core barrel. The quality core specimen, which is retrieved through the system, can then undergo detailed onsite and laboratory analysis.
The tool’s variable volume characteristic allows gas and fluids to expand, while coming out of the hole, without creating an extreme pressure build-up, which would result in unnecessary safety risks, Fig. 1. The tool is designed to run on both a conventional or wireline coring platform. In both of these applications, the core is cut, and, prior to tripping out of the hole, the core barrel is closed and sealed to retain all the gases and fluids. A single storage canister, or set of them, is positioned in the BHA, directly above the core barrel, to capture any expelled gases and liquids that are released from the core, during the trip out of the hole, Fig. 2. The tool is configured, such that overpressure is maintained on the core, to prevent the complete escape of the pore space fluids, until a surface depressurization process can be accomplished, in a controlled manner, at the wellsite.
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| Fig. 1. Overview of the QuickCapture BHA and outer barrel. |
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| Fig. 2. The QuickCapture assembly. |
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The tool is outfitted with pressure and temperature transducer modules, which are meant to record the tool’s behavior. These transducers are placed directly above the core collection chamber, and inside any of the associated storage canisters. The transducers record the trip in the hole, the coring process, the trip out of the hole, and all surface handling activities.
Handling procedures, once the tool reaches the surface, have proven to be a critical step when recovering the core, and associated gasses and fluids. It is crucial that the surface handling kit is fit-for-purpose, based on the targeted reservoir application. Strict handling procedures must be implemented, to ensure that all of the required data are accurately measured, collected and documented.
There are several different methods being practiced on how to recover gases and liquids from the core, depending on the application. In all cases, pressure is released from the barrel, with the associated gases and liquids being measured and collected for further laboratory analysis. Once all of the pressure is relieved from the tool, the core is extruded from the barrel, transferred to a shipping canister, and sent to the laboratory.
PROVEN APPLICATION
The QuickCapture technology has proven most effective when used in tandem with Corpro’s QuickCore, a wireline coring platform designed to optimize all coring operations while obtaining continuous core samples throughout the coring process. This is done without the need to trip pipe, to retrieve the inner barrel assemblies, which house the core. This coring solution is meant to deliver all the efficiencies associated with a wireline retrieval process that minimizes drilling disruption and maximizes coring success, by providing superior core quality and recovery.
Running the two technologies on the same wireline platform allows the client to cut 3-in., wireline-retrieved conventional cores up to 90 ft in length, in addition to 10-ft pressurized cores. An additional feature to this suite of coring tools is the ability to deploy a wireline-retrieved drill insert that can drill ahead, between coring zones. Combining all three of these services offers better value than conventional technologies, in shale applications, due to the thickness and depths of these types of reservoirs.
Recently in the U.S., QuickCore and QuickCapture enabled the successful completion of the largest combined project, where 465 ft of core were recovered. This was in addition to six successful QuickCapture runs with 100% core recovery in both applications, demonstrating both efficiency and cost-effectiveness.
QUICKCAPTURE IN PRACTICE
The first commercial deployment of this technology took place in the emerging liquids-rich Duvernay shale, with a project that began in November 2011. In the Duvernay shale, the technology was utilized for a sequence of exploratory assessment wells, to capture reservoir rock and fluid samples in-situ, to precisely quantify the reservoir properties and hydrocarbon composition. The study’s goal was to gain insight toward the mechanisms by which oil moves through nanoporous mediums, and develop a methodology for quantifying resource-in-place and PVT solutions ahead of future completion efforts.
The operator opted for QuickCapture, to extract core material from the earth, capture all reservoir fluids without losses, and cryogenically preserve the core so that the remaining fluids could be extracted in a laboratory environment, without losses. This would enable scientists to quantify the amount of hydrocarbons, and recombine the fluids and solve for phase behavior.
In the case of the Duvernay shale fairways, conventional and controlled pressure coring operations were performed, based on assessment data collection packages that were prepared for a sequence of multiple wells. Locations were selected across a range of depth, pressure, and reservoir fluid phase windows, from dry gas to more liquids-rich portions of the play, Fig. 3.
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| Fig. 3. An illustration of the field study based on the Duvernay shale. |
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The ultimate goal was to arrive at a re-combined hydrocarbon composition, to reach a “time-zero” data point for use in studies of produced fluid compositional variability. To achieve this, light hydrocarbons were captured, via the controlled pressure core tool and canister desorption, with the heavier hydrocarbons being captured via solvent extraction. These two hydrocarbon sources were then re-combined for the total composition. The resulting fluids were modeled at reservoir conditions and compared to produced fluids taken on surface, from the wellhead.
The coring programs consisted of cutting 3-m controlled-pressure cores, in addition to conventional 18-m core runs, in the zones of interest. In addition to the controlled-pressure cores, PVT and desorption samples were collected. This was for the purpose of comparing recombined fluid compositions, which were determined by using conventionally collected core, to those determined by using samples taken in the controlled-pressure core section, Fig. 4.
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| Fig. 4. The analysis of hydrocarbons from cored material. |
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The conventional core was sampled, at several depth intervals, for tight rock analysis (TRA), source rock analysis, and X-ray diffraction (XRD). A smaller sub-set of the TRA samples was then selected for focused ion beam scanning electron microscopy (FIB SEM) analysis. The data and integrated results were then used to demonstrate the match between modeled petrophysical parameters and the crushed rock TRA data. FIB SEM analysis confirmed indications that the porosity for the Duvernay shale results entirely from the conversion of organic material. Furthermore, 3-D volumetric analysis yielded an excellent correlation between the measured core properties and the associated petrophysical ties.
The combined data sets served as an internal standard by which the effectiveness of the controlled pressure coring results could be benchmarked. In addition, the wellbore was logged, to measure reservoir petrophysical parameters.
Capturing critical resource-in-place parameters, and conducting quantitative analysis of the volatile hydrocarbons contained within the cored intervals, required the development of new field and laboratory methodologies. These procedures helped to develop a workflow capable of directly capturing and analyzing reservoir hydrocarbons from the core material. Field operations and core preservation protocols were developed to maintain the integrity of the hydrocarbon system, during both shipping and core processing back at the laboratory.
Strict on-site handling procedures for the QuickCapture barrel and conventional cores were implemented, to ensure that all required data were accurately documented and measured. These rigorous procedures were followed, since common mistakes in the field—such as poor sample quality, inaccurate data, or missing data—often cannot be overcome.
Upon retrieving the controlled pressure core barrel at surface, the barrel was brought to a stable temperature and connected to a pressure manifold, to safely manage the pressure in the barrel. The manifold was connected to a gas/fluid separator, followed by a series of metering valves, and terminated at a volumetric measuring device. In addition to measuring the volumetric total of gas evolved from the core, additional samples were taken for compositional analysis. After the pressure was terminated, from the core barrel, down to atmospheric conditions, the core was extracted and processed into equal lengths, and sealed in canisters for long-term desorption and gas sampling from the core.
A subset of the core sections, from the conventional cores, was then sealed in vessels and placed in coolers that were packaged with sufficient quantities of dry ice to maintain temperature during shipment to the lab. The freezing process has been used effectively in the past to preserve volatile hydrocarbons in the core material.
Once the laboratory received each of the frozen cores, the sections were removed quickly from their containers and slabbed, using liquid nitrogen cooling. The slabbed section was set aside for geological description. Sections of the remaining butt section were taken from different depths for further laboratory analysis, as referenced above, in addition to sampling any heterogeneity in the core.
FACING CHALLENGES
A number of key challenges were presented on the project, including timing of the spudding of assessment wells, the development of the tool, and casing constraints of the planned wells. Additionally, the timing of the initial opportunity was tight, and the tool had to be adapted from its original wireline platform design, to a conventional platform application, to fit the client’s desired hole size.
Of the five initially planned wells, only four were ultimately executed. While all four of the test pilots were successful at retrieving 100% core recovery in the pressure cylinder, only the fourth and final attempt was successful at executing the trip into the hole; the cutting; the capture of core material; sealing the tool; tripping out of the hole; and extracting the volumetric and compositional data required for analysis.
Over the course of the four-well program, the tool underwent several design modifications. The coring company’s design team was able to make a series of initial design changes to have the tool available for the first well. Thus, the technology’s versatility was tested, from the first application, and evolved, based on real-world implementation, throughout the assessment process.
Additional focus was placed on communication and the hand-off process, between the coring company and the laboratory core analysis vendor. The controlled-pressure core barrel, and the associated gas storage canisters, are dynamic systems responding to extreme changes in temperature and pressure, from downhole to surface environments. Transportation restrictions prohibited any trucking of the coring assembly to an offsite laboratory, so it was required that all gases and fluids needed to be removed from the pressure tool on-site. Numerous processes were envisioned, and several were utilized, for extracting the fluid from the gas accumulators and the core barrel. Due to the small volume of fluid and gas, and the potential for error, a stringent workflow was developed to guide all onsite sampling.
Another key takeaway from the Duvernay implementation was that constraints related to hole and platform should be addressed as early as possible in a client’s program.
RESULTS
The overarching idea behind this technology is simple and intuitive: measure initial fluid composition and contrast it against the produced fluid compositions to determine production.
By extracting the fluids from the rock; by sampling the produced fluids from the separator or wellhead during initial flowback and sequentially over time; and by analyzing the composition of these fluids and observing their variance over time, we can quantify key oil recovery parameters.
Additionally, the program confirmed that the tool can execute successfully on both conventional and wireline platforms.
Results from the extracted fluids, and what was produced from the well, indicate a clear correlation in production, signifying a successful recombination and an accurate representation of the extracted hydrocarbons via controlled pressure coring and solvent extraction.
The deployment of the QuickCapture technology, in the Duvernay shale, demonstrated that the described workflow for controlled pressure coring is the only technology capable of capturing reservoir rock, while retaining all associated gases and fluid material by transporting them to surface in a safe operating environment. 
ACKNOWLEDGEMENTS
This article is based on SPE paper 167199, “Novel controlled pressure coring and laboratory methodologies enable quantitative determination of resource-in-place and PVT behavior of the Duvernay shale,” 2013 SPE Unconventional Resources Conference, Calgary, Canada, Nov. 5–7, 2013.
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