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Considering Time and Space in Drilling and Completion Can Reduce Well Interference

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Development of organic shale reservoirs with large hydraulic-fracture treatments not only poses challenges to the completion of a single well, but also to interference with surrounding wells. A direct consequence of interference is production loss. Therefore, the drilling and completion schedule for field development must be 4D in time and space to account for interaction in between wells. The complete paper describes a physics-based model of interference and a sensitivity study to propose guidelines for well spacing and a drilling timeline for multiple horizontal wells in the Vaca Muerta shale.

Introduction

Effective field development relies on the tradeoff between the capital expenditure (CAPEX) and the production profile generating cash flow. Both CAPEX and the production profile are directly controlled by the number of wells and the pace at which these wells are tied in to the production facilities. With respect to tight rocks, the need for horizontal drilling and hydraulic fracturing adds a layer of geometrical complexity to unlock such resources.

In very-low-permeability reservoirs, the drainage area is controlled by the hydraulic-fracture geometry. However, the relatively large size of the fracturing treatments performed in a multistage manner across several different perforation clusters within a same stage makes hydraulic fractures difficult to control. In practice, fracture dimensions are not constant along a lateral. Additionally, because limited contribution of the matrix is expected in a nano-Darcy environment, the spacing between laterals tends to be smaller than the reach of the hydraulic fractures. This has been evidenced by the increasing concern over fracture hits in the industry, and already has been experienced in the Vaca Muerta.

Interference can be caused by fracture hits while completing the well, competition for drainage area during production, or fracture-geometry deterioration caused by stress field variation when infill-drilling a child well near a produced parent well. Even if some overlap between the hydraulic fractures of two neighboring wells is advisable to avoid leaving reserves behind, the tradeoff between well spacing, completion intensity, and production interference remains a delicate exercise. Hydraulic fracturing should be considered at the field level, rather than independently for each well, giving completion a multiple-well spatial dimension.

As an additional complexity, child wells completed later in the development of a lease tend to perform more poorly than the parent wells. This performance impairment might have several different sources, but one of the most-accepted hypotheses is that the heterogeneous contrast in the stress field generated by the depletion around the parent negatively affects the contact area provided by the hydraulic fractures of the child well.

Reservoir-Centric Vaca Muerta Work Flow

The Vaca Muerta shale, located in Argentina in the source rock of the Neuquén basin, ranks among the most-promising shale prospects outside the US. After the first exploratory well was completed in 2010, development of the play has continued at a constant pace. Still, only a few blocks have started their full field development, and the total number of producing wells remains an order of magnitude lower than that seen in most active shale plays in the US.

The Vaca Muerta faces a variety of challenges, including geology, operations, production, planning, and commercial development. Accelerating the learning curve is critical to ensuring success, and taking advantage of the learnings from other active shale plays—including placing the well completion into the space- and time-dependent context of the field development—is key.

The paper presents a detailed discussion of a reservoir-centric, fracturing-to-production work flow that begins with a detailed characterization of the reservoir that is input into a hydraulic-fracture simulator generating nonplanar geometries that are later explicitly gridded and fed into a reservoir simulator to evaluate the resulting production. The high degree of integration of the work flow allows correlating the production effect with reservoir properties or completion scenarios for both single- and multiple-well configurations.

The work flow integrates all steps from well construction to production, including reservoir petrophysics and geomechanics, nonplanar hydraulic-fracture geometry, and production simulation with fit-for-purpose simulators. A multiple-well model enables investigating possible hydraulic-fracture overlap between laterals and competition for drainage as a function of the well spacing. Reservoir depletion can be tracked effectively, and corresponding stress field variations are estimated using a 3D geomechanical finite-element simulator. The hydraulic-fracture geometry and production of a child infill well is simulated on the basis of this updated stress field, in which a modified stress contrast has been created by the depletion of the parent well.

By running different scenarios, competition for drainage between wells is evaluated quantitatively to balance individual well performance and field recovery. Factors affecting well spacing such as lateral landing, stress profile, ­hydraulic-fracture geometry, and number of pay intervals are investigated. An example graph is provided in Fig. 1. Well spacing acts as an additional geometrical constraint during completion. Its effect on the completion optimization process when compared with that of a standalone lateral is discussed. A time-dependent relation between infill-drilling of a child well and the production loss caused by the effect of stress alteration near a producing parent well is derived. Recommendations for well spacing and completion modifications are provided to minimize the production loss of the child well at different stages of the production of the parent well.

Fig. 1—Comparison of well spacing for the different lateral landings of Middle and Lower Vaca Muerta (VM).

 

The proposed approach places the completion of each well in the field-development context by considering a 4D reservoir-depletion, geomechanics, and hydraulic-simulation coupling. The methodology enables a quantitative effect on production along with practical drilling timeline and completion recommendations when planning for multiple wells in a field development.

Observations

  • Multiple-well modeling, based on reservoir characterization and completion design, allows establishing a correlation between the individual performance of a well and the spacing to its neighbors. This correlation can be upscaled to a multiple-well scenario per area and fed into an economic model to determine optimal well spacing to maximize returns.
  • Optimal well spacing depends on both hydraulic-fracture geometry and economic parameters. Varying the lateral landing point, stress containment, well cost, or oil price might call for different well spacing. Just as spacing should be considered horizontally, the staggering of laterals in a multiple-pay development in thick formations such as the Vaca Muerta might lead to a reduction of individual well performance. Stacking a larger number of laterals per unit surface in a multiple-pay scenario, even at the cost of some individual performance reduction, might lead to higher return if the cost-benefit tradeoff is engineered adequately.
  • Well spacing is an additional geometrical constraint that should be considered in the completion design. Even if increasing proppant and fluid density might be beneficial regardless of the constraint of the spacing, perforation-cluster spacing might differ if the lateral is assumed to be standalone or surrounded by neighbors at a fixed distance.
  • Depletion during producing affects the stress field around the well, particularly in the high level of drawdown observed in the highly overpressured Vaca Muerta. This depletion-induced stress reduction creates a pressure and stress sink around the parent well, negatively affecting the hydraulic fractures created by the completion of a child well drilled later in its vicinity. Production impairment of the child well caused by its proximity to a producing parent well might be in excess of 40% reduction of the parent well production after 1 year and 30% after 10 years.
  • The negative effect of the parent depletion over the child production might be notable even if the delay between the parent tie-in and child completion is on the order of 1 year. Still, the early production of a lateral in shale formation is complex, and initial overpressure during flowback is commonly observed but not considered in the modeling, and might delay the effect of the parent depletion over the child completion and production.
  • A good assessment of the parent-well hydraulic-fracture geometry is necessary to estimate its drainage area and possible remedial measures that could be applicable for the child-well completion. Attempting to let the pressure build up by shutting in the parent before the completion of the child well to reduce the depletion-induced stress contrast seems to have little effect. However, increasing the distance between the parent and the child wells might effectively reduce the level of production impairment of the child well.
  • During the completion of the child well, some of the hydraulic fractures grow into the area of influence of the parent well. Given that a pressure gradient toward the parent well is already established, some of the new fracture surface profits the parent well rather than the child well and might enhance the production of the parent. Still, this production increase remains marginal and does not offset the loss of the child well, leaving the pad balance negative.
This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 191836, “When, Where, and How To Drill and Complete Pads of Multiple Wells? Four-Dimensional Considerations for Field Development in the Vaca Muerta Shale,”by Stephane Pichon, SPE, Federico Gaston Cafardi Orihuela, Emilio Lagarrigue, and Gustavo Cavazzoli, SPE, Schlumberger, prepared for presentation at the 2018 SPE Argentina Exploration and Production of Unconventional Resources Symposium, held in Neuquén, Argentina, 14–16 August. The paper has not been peer reviewed.

Considering Time and Space in Drilling and Completion Can Reduce Well Interference

01 September 2019

Volume: 71 | Issue: 9

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