Integrated Completion Sensitivities for Horizontal-Well Design in the Vaca Muerta
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The boom in organic shale plays has revealed the critical need to size hydraulic-facture treatments correctly to achieve commercial success. The right balance must be found between the cost of fracturing and the additional production achieved by increasing the formation-to-wellbore contact area. The complete paper examines a range of completion scenarios to evaluate the relationship between hydraulic-fracture design, production, and well profitability by use of numerical simulations to guide completion of horizontal wells in Argentina’s Vaca Muerta Shale.
The Vaca Muerta Shale is the source rock of most of the producing formations in the Neuquén Basin, with high potential as a standalone reservoir. The first well aiming at testing production from the play was drilled and completed in 2010, and, at the end of 2016, the production from the formation involved more than 600 wells. Well-construction practices have moved from creating vertical wells to creating horizontal wells.
In any organic shale play, completion has a significant weight in the total well cost and must be sized adequately. Completion design is composed of the volume and number of hydraulic fractures to be created along the lateral, and it must be engineered according to the specific features of each formation.
One of the principal features of an organic shale reservoir is the absence of commercial production unless the well is hydraulically fractured. Enhancing the contact area between the formation and the wellbore with hydraulic fractures compensates for the extremely low permeability of these formations. Multiple factors, including geomechanics and stress direction, drive the geometry of the hydraulic fractures. However, it must be noted that most organic shale reservoirs are overpressured. This factor tends to drive the stresses up and reduce their horizontal anisotropy.
Besides the hydraulic-fracture geometry, production is also the result of the hydrocarbon volume in place and the interaction of the flow capacities between the reservoir and the created network of conductive fractures. Therefore, the size of this surface of exchange created by the hydraulic fractures has a critical effect on the level and dynamics of the production profile.
Finally, economics is the balance between total well cost and production. The completion design affects both production and total well cost, making it a critical parameter.
Fracturing-to-Production Integrated Work Flow
Work-Flow Requirements. With the primary objective being the combined evaluation of completion design, hydraulic-fracture geometry, production, and economics, the methodology must allow an explicit description, at either the input or output level, of each one of the aforementioned parameters. The fracturing-to-production work flow fits this purpose and is proposed for this study. The work flow has been implemented successfully in the Vaca Muerta and various other shale plays.
Reservoir Characterization. The inputs for the work flow constitute a detailed characterization of the static properties of the reservoir, including petrophysics, geomechanics, and a description of the natural fractures. All the static properties are explicitly described vertically across the entire thickness of the section of interest and are propagated horizontally over the entire volume of the model.
Data specific to the Vaca Muerta Shale are taken from the literature and summarized in Table 1 of the complete paper. Average values are used to populate the static model and ensure a generic but consistent representation of the areas currently under development.
Hydraulic-Fracture Modeling. An accurate modeling of hydraulic fracturing is a key element in this study to evaluate the resulting geometry against various possible completion designs. The unconventional fracturing model (UFM) is chosen as the hydraulic-fracture model. The main features of the UFM model include the following:
- It is a physics-based model that honors elastic deformation and material balance.
- It dynamically evaluates the distribution of fluid and proppant between different perforation clusters of a single stage.
- It models the transport of proppant along the fractures.
- It includes the interaction of natural fractures, to create nonplanar hydraulic-fracture geometries.
- It includes the effect of stress shadows generated within and between fracturing stages.
It is important to note that, because of all the mechanisms taken into account in the UFM model, the actual geometry of each hydraulic fracture along the lateral varies according to its surroundings and the sequence of the operation, as shown in Fig. 1. Therefore, a statistical description of the hydraulic-fracture dimensions is preferred and allows assessment of a range of possible variations along the lateral for a given completion design.
Reservoir Simulation. Once the hydraulic-fracture simulations of all stages are performed, the resulting geometry is merged with the background properties of the reservoir to create the grid to be used in a reservoir simulation. The grid is unstructured and allows explicit characterization of the conductivity distribution along each hydraulic fracture. An additional advantage of being able to describe the hydraulic fractures completely is the characterization of the propped and unpropped areas. The model is split into three different regions covering the formation matrix, the propped fractures, and the unpropped hydraulic fractures over which specific conductivity-degradation laws can be applied independently to represent the dynamics of each element of the system better.
Reservoir fluid is defined by a black-oil model applicable to both light oil and dry gas, and the simulation is controlled by an imposed bottomhole-flowing-pressure profile. Data describing the dynamic properties of the Vaca Muerta are gathered and summarized in Table 2 of the complete paper. Average values are used to populate the model. The production profile is combined with well cost and operational expenditure to evaluate economics.
Completion Sensitivity Study
A total of 60 completion scenarios were run to create a database covering more than 550 fracturing stages and more than 1,600 individual hydraulic fractures. To compare different cases, the following indicators are considered in the sensitivity study:
- Long-term recovery (10-year cumulative hydrocarbon)
- Short-term recovery (first-year cumulative hydrocarbon)
- Net present value (NPV)
Hydraulic-Fracture Geometry and Production. The current methodology, by defining every hydraulic fracture explicitly, enables accessing different levels of detail. Hydraulic-fracture properties can be characterized at the cluster level, stage level, or well level. The level of detail must be adjusted to fit the purpose of each study. With the objective of comparing completion and production, values averaged at the well level are preferred in this work. Still, the dispersion of the obtained simulated geometries remains of interest in determining the effect of completion design over the hydraulic-fracture geometry.
The resulting production of each completion scenario is compiled and compared. Overall production can vary up to 40% from its baseline. The major changes occur in roughly the first year of production, both positively and negatively. The most significant effect is that seen in the production rate, in which worst cases tend to reduce the initial rate and delay production whereas best cases speed up hydrocarbon recovery and sustain a higher rate for longer.
Proppant Volume. Total proppant per lateral length shows a good correlation with all the proposed indicators (early and long-term production and NPV). A larger volume of proppant pumped correlates with a larger propped surface. A larger propped surface then translates to a higher initial production and sustains the production rate longer.
Proppant Concentration. A high proppant concentration might provide an excess of conductivity of the proppant pack or reduce the overall fracture surface by reducing the fluid volume for the same amount of proppant. However, higher proppant concentration has an effect on the amount of surface that remains propped out of the total surface created. Increasing the propped fraction of the total surface helps in increasing the propped height. A similar observation can be made along the length of the& fracture.
In practice, a given proppant concentration implies an adjustment in fracturing-fluid viscosity to ensure successful placement. Therefore, the effect of proppant concentration cannot be isolated from the type of carrier fluid and its transport ability.
Cluster Spacing. Cluster spacing shows a correlation with the initial production, but no relation with either the long-term recovery or the NPV. Reducing the spacing of the perforation clusters allows increasing the total number of hydraulic fractures per lateral length and increasing the total created fracture surface. However, a tighter spacing increases the fracture-propagation pressure because of a stronger stress shadow, which possibly affects the final fracture geometry negatively by increasing the requirements for limited entry to divert fluid efficiently between all the perforation clusters of a given stage.
A tighter cluster spacing is usually obtained by increasing the number of perforation clusters treated within the same fracturing stage. A larger number of perforation clusters reduces the probability of an even distribution of fluid (and proppant) among the clusters of a given stage.
Fluid Volume and Type. As expected, a good correlation can be observed between the total amount of fluid pumped and the total hydraulic surface created. However, no clear correlation appears when comparing total fluid volume and the total propped surface. Large fracturing treatments usually include mostly low-viscosity fracturing fluids such as slickwater to maintain the completion cost. This practice has been taken into account in the considered completion scenarios by increasing the fraction of low-viscosity fluid as the volume of fluid increases. No clear correlation seems to appear between the volume of fluid pumped and the early production or NPV. Eventually, a larger volume of fluid pumped creates a larger hydraulic surface but without conductivity sufficient to sustain long-term production.
Integrated Completion Sensitivities for Horizontal-Well Design in the Vaca Muerta
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