Improved completion design and field development strategies have provided commodity price resilience by sustained efficiency gains across most major US Shale plays. This rapid evolution in completion practices, however, has created behind pipe opportunities. Refracturing offers a viable solution to maximize on these opportunities, however, its effectiveness is dependent on a variety of factors. The present paper explores the implementation of refracturing as a re-development strategy in legacy shale plays and evaluates it as a truly multivariable problem.
The paper takes into consideration petrophysical parameters, initial completion design, chemical composition, formation quality, time from original completion, refrac completion design and production performance to quantify impact on refrac KPIs such as IP ratio, EUR ratio, decline trend impact, amongst others. The paper does this by using an ACE (alternating conditional expectation) non-linear regression model that incorporates the KPI’s as response variables and utilizes the transforms of a wide range of input variables to identify cause and effect relationships. By running this analysis across multiple legacy shale plays, including the Haynesville, and Barnett, the paper provides best-practices to maximize refracturing success.
While refrac can offer a viable solution in obtaining incremental production, depending on the basin, a refrac can be a tenth of the expense of a new well and can beneficially impact the production from the existing well. In most cases, the analysis found EUR predictions improved by 30% - 200%. While correlations varied across basins and completion design, an inverse correlation was found between refrac KPIs and initial frac intensity.
Although, refracturing in horizontal shale wells is a well-established practice, a significant amount of analysis on their performance is focused on one or two key variables. The present paper adds to the existing body of literature by using data analytics and machine learning to evaluate this strategy from a truly multivariable standpoint. The paper also provides best practices to evaluate and predict refrac performance to de-risk refrac as a field re-development strategy.
Relative permeability has a significant impact on gas or oil and water production, but is one of the most complicated properties in unconventional reservoirs. Current understanding on relative permeability for unconventional reservoir rocks is very limited, mainly because of a lack of direct measurement of relative permeability for these rocks that have matrix permeability of sub-micron-Darcy level. Due to the difficulties related to the direct measurement, most studies on relative permeability in unconventional reservoirs are based on indirect or modeling methods. In this paper, a modified gas expansion method for shale matrix permeability measurement (Peng et al., 2019a) was adopted to measure gas relative permeability directly under the scenario of water imbibition for samples from different unconventional reservoir formations. Evolution of gas permeability, along with gas porosity and fracture-matrix interaction, during the process of water redistribution (mimic of what occurs in shut-in period in real production) were also closely measured. Results show that gas relative permeability in matrix decreases during water redistribution because of water imbibition from fracture to matrix and water block effect. Water block effect is more significant at low water saturations than higher water saturations, leading to a rapid-to-gradual drop of gas relative permeability with increasing water saturation.
A conceptual model on water redistribution in a fracture-matrix system and the change of gas and water relative permeability is proposed based on the experimental results and observations. Influencing factors including pore size, shape, connectivity, and wettability are taken into account in this conceptual model. The combined effect of these four influencing factors determines the level of residual gas saturation, which is the most important parameter in defining the shape of relative permeability curves. Water relative permeability is predicted based on the conceptual model and the measured gas relative permeability using modified Brooks-Corey equations. Deduction of oil-water relative permeability is also discussed, and experimental methods on determination of the key parameter, i.e., residual oil saturation, are proposed. Implication of relative permeability on gas or oil and water production and potential strategy for optimal production are also discussed in the paper. Hysteresis effect is not included in this study and will be addressed in future work.
Connectivity of the pore system is crucial for production of hydrocarbons from unconventional resources. In shales, pore throats critically control and limit permeability. Even if larger pores are the dominant pore size, small pores throats could ultimately control the access to that pore space. Mercury injection capillary pressure (MICP) measurements are commonly made to determine pore throat size distributions. Results for shales usually show large injection volumes associated with pore throats just several nanometers in diameter. The existence of these small pore throats has also been confirmed by Focused Ion Beam/Scanning Electron Microscope (FIB/SEM) analysis. One of the unique properties of mercury is that it is non-wetting to both matrix phases present in organic-rich shales; therefore, it can access pore systems in both organics and inorganics. MICP measurements dynamically alter the pore structure through pore compressibility which intrinsically depends on the aspect ratios of the pores; crack like pores, with very high aspect ratios, may close at low pressures and may not be sampled by MICP. The connectivity of the pore space and how much of it is accessed by MICP remains poorly understood.
Here we report on shale samples that have undergone MICP followed by Micro X-ray Computed Tomography (μXCT) and FIB/SEM imaging. μXCT results show that not all regions of the shale samples were accessed uniformly by MICP. Mercury is observed going into fractures and penetrating into the shale matrix. The distance away from the fractures and the percentage of the sample volume accessed by mercury has been calculated. Some samples, such as the Tuscaloosa Marine Shale, showed mercury penetration throughout specific layers in the sample, whereas Eagle Ford samples showed mercury penetration more uniformly and on average of almost 150 μm away from the fractures with almost 60% of the entire sample volume accessed by the mercury. These μXCT results suggest that mercury is not fully accessing all the pore space of the sample even at 60,000 psi which corresponds to a pore throat radius of 1.8 nm.
Cryo FIB/SEM was used to further investigate mercury intrusion into the shale matrix at the nanometer scale. Frozen droplets of mercury were observed in pores as small as 30 nm which corresponds to an injection pressure of 6,000 psi. The mercury clearly accessed the organic pores and remained after pressure was reduced. This is also reflected in the hysteresis observed in the MICP spectra captured during pressurization and depressurization. The magnitude of the hysteresis is a consequence of the differences between pore bodies and pore throats. Like the μXCT, SEM results show that intrusion of mercury into the sample is not uniform indicating that many of the pores are not connected to the outside of the sample. These results suggest that pore connectivity in shales may be very limited, and the volume accessible may not extend far from fractures in the shales.
Determination of ideal horizontal targets for unconventional reservoirs often necessitates an understanding of the reservoir from the global tectonic to the sub-microscopic scale. When selecting a target zone, it is necessary to consider the abundance, composition, and delivery of sediment to basins; the production, preservation, and alteration of organic matter; and the diagenetic and structural modification of the stratigraphic section. Here, we focus on two sedimentologic phenomena common to the Marcellus Shale of the Appalachian Basin of southwestern Pennsylvania. Namely, we explore the strategy of targeting high organic carbon/biogenic silica facies and the challenges posed by encountering carbonate concretion horizons.
Geochemical observations including Si/Al and Si/Zr, and thin section and scanning electron microscopy indicate abundant recrystallized biogenic quartz cement in the Marcellus Shale. Burial models suggest that prior to the end of mechanical compaction; the Marcellus entered the oil window, and presumably began generating organic matter-hosted porosity at a depth of ~1200m. Notably, at similar organic carbon content, samples with elevated biogenic silica yield higher porosity and permeability. These observations suggest that biogenic quartz may play a role in the deliverability of hydrocarbons by providing a compaction resistant framework conducive to the preservation of organic matter-hosted pores and pore throats. Further, biogenic quartz-rich facies demonstrate increased rates of penetration allowing for more efficient drilling of laterals.
However, carbonate concretions encountered while drilling horizontal Marcellus Shale wells negatively affect drilling operations by reducing drilling rates, damaging bits, and requiring excessive steering corrections to penetrate or extricate the bit from the horizon. Carbonate concretions form by the anaerobic oxidation of methane in a narrow zone perhaps just a few meters below the seafloor. Crucial to this mechanism is a slowing or pause in sedimentation rate that would have held the zone of carbonate precipitation at a fixed depth long enough for concretions to grow. Using this model, we attempt to predict the size and location of concretions to avoid encountering them while drilling. Field observations of Upper Devonian shale-hosted concretion dimensions suggest that Marcellus-hosted concretions up to three feet in length are possible. Hiatuses in sedimentation and potential concretion horizons were predicted using uranium to organic carbon ratios. The attachment of uranium to organic carbon macerals occurs across the sediment-water interface. Therefore, an increase in the abundance of uranium per unit organic carbon indicates a cessation in sedimentation and the potential for concretion growth. Indeed, when comparing well log response to core, uranium to organic carbon excursions predicted the location of two concretion horizons.
Objectives/Scope: Understanding that capillary forces will act to limit petroleum fluid saturations in water-wet fine-grained rocks, including organic rich source rocks, dates back at least to Hubbert (1953). Likewise, Philippi (1965) noted relationships identifying sorption in/on organic matter as a significant storage mechanism in organic-rich rocks. Contrast these early insights with current unconventional reservoir evaluation, where we observe a disconnect between in situ (core exhumed to surface) measured total water saturations vs. the produced cumulative water volumes from a given stimulated rock volume. Water-free production in gas shales, from gas-wet organic matrix pores, created an early impression that unconventional plays don't produce water. So, in more liquid-rich plays, water cuts were initially under-appreciated: e.g. >80% in the Wolfcamp (stock-tank basis). If measured Sw is so low (core-based calibration), where is the water coming from; or is there an alternative method to more accurately relate in situ to produced water and petroleum production?
Methods/Procedures/Process: Adapting organic sorption models from the 80's, we can split total hydrocarbon volatiles into sorbed and, by difference, non-sorbed (fluid phase) yields. Converting to volumes and adding back dissolved gas using a formation volume factor (FVF) we can estimate the bulk volume fluid phase. This new approach then yields observations regarding remaining water-filled pore volume versus sorbed and non-sorbed hydrocarbon volume explaining the high water cuts in the Permian Basin stratigraphy; and additionally may indicate sweet spots in pore systems in different parts of the rock compared to alternatively derived saturations.
Results/Observations/Conclusions: The final piece of the puzzle comes from basin modeling of petroleum charging in the 90's. Some scientists applied conventional reservoir relative permeability to fine-grained rocks, but new research predicted that progressively finer grained rocks with higher irreducible water should be able to flow oil at progressively higher Sw: at 100nD, both oil and water should flow at Sw > 80%. Lower petroleum phase saturations and adjusted relative permeability curves may better explain observed production behaviors and profoundly alter our view of recovery factor and stimulated rock volume.
Applications/Significance/Novelty: The method offers an alternate and independent method to Dean-Stark-based core / SWC saturation analysis and its pitfalls. Saturation patterns after removal of immobile sorbed oil are different to those derived using the Dean-Stark based method, implying sweet spots / landing zones can be further optimized even in maturing shale plays. Lower oil-in-place – representing only the potentially mobile fluid phase petroleum – means that fracture stimulation has a higher recovery factor than previously thought, with profound effects on the infill volumes / opportunities for future field developments and therefore ultimately for US – and global – oil supply projections.
Interdisciplinary Components: Cross-over technology from organic geochemistry to petrophysics to reservoir engineering.
Ursell, Luke (Biota Technology) | Hale, Michael (Novo Oil & Gas LLC) | Menendez, Eli (Novo Oil & Gas LLC) | Zimmerman, John (Novo Oil & Gas LLC) | Dombroski, Brian (Novo Oil & Gas LLC) | Hoover, Kyle (Novo Oil & Gas LLC) | Everman, Zach (Novo Oil & Gas LLC) | Liu, Joanne (Biota Technology) | Shojaei, Hasan (Biota Technology) | Percak-Dennett, Elizabeth (Biota Technology) | Ishoey, Thomas (Biota Technology)
Subsurface DNA is an emerging independent diagnostic offering oil and gas operators a high resolution and non-invasive measurement of fluid movement in the subsurface. DNA sequencing methodologies that use subsurface DNA markers acquired from well cuttings and produced fluids are being increasingly used in the Permian Basin to elucidate drainage heights for new and existing wells with increased temporal and spatial resolution. Drainage height estimates are applied across the asset lifecycle during appraisal, development, and production. We present a new exploratory application for DNA Diagnostics in the Midland Basin as a complementary data set for understanding reservoir characteristics when existing wells and data are not available.
In this work, Novo Oil and Gas and Biota Technology performed a study on an exploratory well in the Meramec formation of Ector County. Well cuttings were collected from a pilot hole to create a vertical DNA baseline through key Barnett and Meramec formations, and from a lateral section to estimate per stage oil and water contribution. Frac fluid was collected during completion and produced fluids were collected through the initial 189 days of production. A data science-based workflow was performed that tracked DNA markers within produced fluids and compared them to a well-cutting derived DNA baseline to estimate per-formation and per-stage contributions in the vertical and lateral sections, respectively. DNA Diagnostic results were integrated into a reservoir engineering workflow through comparisons with petrophysical logs, core data, geosteering reports, completions reports, production data, and oil tracers.
Results showed that initial drainage heights covered a large portion of the Barnett into Woodford formations and corresponded to the higher initial production values. Over time, the DNA drainage heights indicated a focused zone of contribution from the Barnett which corresponded to a steady, flat decline curve. Lateral DNA contributions estimates indicated the highest production contribution from a section of the lateral drilled within the intended landing zone towards the toe, which was corroborated with conventional oil-based chemical tracers. Additionally, the lateral DNA Stratigraphy plots allowed for the development of a hypothesis of a potential fault encountered in the lateral, which subsequent wells will investigate.
Overall, we demonstrate that Subsurface DNA Diagnostics provides an independent workflow to estimate drainage height and lateral production allocation by analyzing DNA markers acquired from cuttings and produced fluids. This work shows the complementary nature of incorporating DNA Diagnostics into traditional reservoir engineering workflows as a hypothesis generating tool and as a corroborative measurement. The scalability and non-invasive nature of the workflow has the potential to improve initial characterization and operations during field development, particularly exploratory areas with less operational history. DNA Diagnostics provided direct economic benefit to Novo's field development plan and informed subsequent capital allocation strategies.
Gupta, Ishank (University of Oklahoma) | Devegowda, Deepak (University of Oklahoma) | Jayaram, Vikram (Pioneer Natural Resources) | Rai, Chandra (University of Oklahoma) | Sondergeld, Carl (University of Oklahoma)
Planning and optimizing completion design for hydraulic fracturing require a quantifiable understanding of the spatial distribution of the brittleness of the rock and other geomechanical properties. Eventually, the goal is to maximize the SRV (Stimulated Reservoir Volume) with minimal cost overhead. The compressional and shear velocities (Vp and Vs respectively) can be used to calculate Young’s modulus, Poisson’s ratio and other mechanical properties. In the field, sonic logs are not commonly acquired and operators often resort to regression to predict synthetic sonic logs. We have compared several machine learning regression techniques for their predictive ability to generate synthetic sonic (Vp and Vs) and a brittleness indicator, namely hardness, using the laboratory core data. We used techniques like multi-linear regression, lasso regression, support vector regression, random forest, gradient boosting and alternating conditional expectation. We found that the commonly used multi-linear regression is sub-optimal with less-than-satisfactory predictive accuracies. Other techniques particularly random forest and gradient boosting have greater predictive capabilities based on several error metrics such as R2 (Correlation Coefficient) and RMSE (Root Mean Square Error). We also used Gaussian process simulation for uncertainty quantification as it provides uncertainty estimates on the predicted values for a wide range of inputs. Random Forest and Extreme Gradient Boosting techniques also gave low uncertainties in prediction.
Euzen, Tristan (IFP Technologies (Canada) Inc.) | Watson, Neil (Enlighten Geoscience Ltd.) | Chatellier, Jean-Yves (Tecto-Sedi Integrated Inc.) | Mort, Andy (Geological Survey of Canada) | Mangenot, Xavier (Caltech)
With the development of unconventional resources, the large number and high density of well data in the deep/distal part of sedimentary basins offer new avenues for petroleum system analysis. Gas geochemistry is a widespread and inexpensive data that can provide invaluable information to better understand unconventional plays. This paper illustrates the use of early production gas composition as a proxy for in-situ hydrocarbon phase distribution in the Montney play of westernmost Alberta and northeastern British Colombia. We demonstrate that a careful stratigraphic allocation of the landing zone of horizontal wells is a key step to a meaningful interpretation and mapping of gas geochemical data. The regional mapping of the dryness of early production gas from the Montney formation clearly delineate thermal maturity windows that are consistent with available carbon isotopic data from produced and mud gas. Integrating this mapping with pressure and temperature data also highlights gas migration fairways that are likely influenced by major structural elements and compartmentalization of the basin. In the wet gas window, reported condensate-gas ratios show that the liquid recovery from multi-stage fractured horizontal wells is highly variable and strongly influenced by variations in reservoir quality and stimulation design. Understanding in-situ fluid distribution can help narrow down the number of variables and identifying key controls on liquid recovery. Several examples combining produced and mud gas data illustrate the use of geochemistry to better constrain geological and operational controls on productivity and liquids recovery in the Montney play.
With the rapid development of unconventional resources, a wealth of new data has been released from historically undrilled or poorly documented portions of sedimentary basins. The large number and high density of well data over extended areas of deep/distal parts of these basins offer invaluable information and new perspectives for petroleum system analysis. In the Montney play of Western Canada, the distal unconventional part of the basin covers an area of approximately 65,000 square kilometers and has been penetrated by over 7,000 horizontal wells. Due to sustained low gas price in North America over the past decade, most of the industry activity has been focused on the liquids-rich gas and light oil fairways of this resource play. Production data show that although a broad liquids-rich fairway can be defined at the basin scale, local variations of fluid distribution and reservoir quality strongly affect the liquid recovery from horizontal wells. The geochemical compositions of both produced gas and mud gas provide a powerful tool to investigate those variations, their geological controls and their impact on well performance. While this paper focuses on the fluid distribution, numerous studies have documented the influence of reservoir quality on the liquid recovery in the Montney play (Chatellier and Perez, 2016; Kato et al., 2018; Akihisia et al., 2018; Iwuoha et al., 2018).
Brice Y. Kim and I. Yucel Akkutlu, Texas A&M University, and Vladimir Martysevich and Ronald G. Dusterhoft, Halliburton Summary The stress-dependent permeabilities of split shale core plugs from Eagle Ford, Bakken, and Barnett Formation samples are investigated in the presence of microproppants. An analytical permeability model is developed for the investigation, including the interactions between the fracture walls and monolayer microproppants under stress. The model is then used to analyze a series of pressure-pulsedecay measurements of the propped shale samples in the laboratory. The analysis provides the propped-fracture permeability of the samples and predicts a parameter related to the quality of the proppant areal distribution in the fracture. The proppant-placement quality can be used as a measure of success of the delivery of proppants into microfractures and to design stimulation experiments in the laboratory. Introduction Unconventional-oil/gas resources, such as tight gas/oil and resource shale, have low porosity and ultralow permeability. Creating a well-connected complex fracture network is a key component of increasing the permeability and accelerating production. The early era of hydraulic fracturing horizontal wells in unconventional formations was concerned with achieving long fractures with multistage treatments with large cluster spacing. However, recent trends in this type of well completion and stimulation involve fractures that are created in narrower clusters in much closer spacing, targeting larger surface areas. It is argued that the practice of hydraulic fracturing with narrow clusters in close spacing along a lateral wellbore creates fractures with significantly reduced sizes, but in a complex network (Rassenfoss 2017). The creation of a network of fractures includes major operational issues.
It was designed to reduce the completion or stimulation cost and time by placing multiple perforation clusters (typically three or more clusters per stage) and performing fracture stimulation by pumping the fluid/proppant system at very high rates to generate the necessary differential pressure and multiple fractures per stage. Because the stimulation pump rate in unconventional reservoirs is typically a function of the number of perforation clusters, number of perforations per cluster, the entry hole diameter of the perforations, and a fixed required minimum rate per perforation, the resulting typical pumping rates to perform fracture stimulations to generate more than one fracture per stage is in the range of 60 to 100 bbl/min or higher. However, historically, the fracture stimulation of multiple perforation clusters performed at high pumping rates often resulted in almost 80% completion efficiency as documented by Wheaton et al. (2016). Therefore, during each stage, single or multiple perforation clusters might not be fractured or marginally stimulated, and thus do not contribute to well production. The same system has been also used during refracturing operations or to increase the number of clusters per stage.