Successful development of unconventional resources boils down to two processes—reservoir stimulation through hydraulic fracturing and hydrocarbon recovery. These two processes are inherently coupled as hydrocarbon recovery depends upon effective stimulation treatment.

The challenge facing the industry is how to evaluate and quantify the effectiveness of hydraulic fracturing. There are several parameters that influence the effectiveness of the stimulation, including wellbore orientation, fracturing sequence, fluid and proppant volumes and rock property variations along the wellbore.

Microseismic-based completions evaluation tools provide insight to answer these questions through a comprehensive and deterministic analysis that measures the proppant-filled fracture volume, and how it varies based on different treatment parameters. This allows operators to make informed decisions when optimising completions to maximise recovery for every well.

One of the key challenges facing the industry is to relate the measured microseismic activity to production and recovery of hydrocarbons from individual wells. Historically, microseismic data has been used to gain an understanding of the boundaries of the stimulated rock volume. The gross dimensions of the stimulated rock volume alone, unfortunately, are poor indicators of well productivity, due to a myriad of factors that impact hydrocarbon production.

To really understand the production potential of each well, it is necessary to understand and quantify the improvements in system permeability generated through the process of hydraulic fracturing. The degree of improvement in permeability will eventually dictate the production rates and ultimate recovery. Permeability is a key parameter for hydrocarbon production and reservoir management and can help improve the efficiency and effectiveness of completions. Field development, well placement, hydraulic fracture design and the choice of improved and enhanced recovery methods are all impacted by reservoir permeability.

Productive-SRV and permeability

The basis for completion evaluation involves acquiring and processing microseismic data during hydraulic fracturing to identify where fractures occurred, the magnitude of those fractures and the direction in which the rock broke for each fracture. From there, a magnitude-calibrated discrete fracture network (DFN) is computed to map the orientation and size of each fracture.

Based on this magnitude-calibrated DFN, a mass balance model is used to determine what proportion of fractures contain proppant and should, therefore, be productive. The resulting productive discrete fracture network (P-DFN) and productive stimulated rock volume (Productive-SRV) allow quantifying optimal well spacing, stage lengths and treatment efficiency.

Analysis of this propped fracture volume provides a method to directly compare the effectiveness of various treatment options by measuring the effective fracture height, fracture length and propped volume distribution away from the wellbore.

In order to obtain microseismic derived permeability (an estimate of how readily oil and gas will flow through the rock), the SRV and P-SRV volumes can be subdivided into a 3-D cellular grid. Based on the number of fractures, orientation of fractures and other attributes within a given cell, a permeability estimate is calculated for the rock volume containing microseismic activity.

A permeability scalar is then computed using the range of permeability values computed for each cell within the P-SRV (Figure 1). The scalar allows operators to understand how permeable each cell is in relation to other cells—or which portions of the wellbore have the highest and lowest permeability.

After estimating the amount of permeability for each grid cell, the resulting permeability distribution, together with the system permeability, can be used as input into a reservoir simulator to predict gas/oil production from each well (Figure 2).

Completions optimisation

Operators are focused on finding answers to optimise completions. Using Productive-SRV and PermIndex many crucial questions can be answered to help understand and improve the effectiveness of the treatment. Some common questions that continue to be asked and assessed through microseismic are:

  • How can we quantify the productivity potential for every stage?
  • Does the sequence of zipper fracking have an impact on the SRV?

Correlation to production

In Figure 3, permeability is computed on a stage-by-stage basis using the microseismic events observed during the hydraulic fracturing of each stage. Conceptually, all other things being equal, a stage with higher bulk permeability will produce more fluids than a stage with lower bulk permeability.

A production log was run in the target well to obtain an instantaneous production profile. The example shows the comparison of the measured production with the microseismic derived permeability. In general, we see that there is a good match between stages with higher computed permeability and stages with higher production. This suggests that the microseismic derived permeability can be used as a potential indicator of production from a given stage. The analysis only captures the productivity potential at the early stages, and does not capture the time or pressure dependent changes in production that can be expected with continued production over longer periods of time.

Comparison of the microseismic derived permeability with other geological and well placement information, such as gamma ray, porosity, distance of wellbore from a boundary and rock brittleness, can help understand the differences in individual stage production. This will help improve the treatment and well placement for future wells in the same area and could also help identify and rank stages for potential workover in the future.

Production logs are not usually acquired in most unconventional wells due to cost, logistics and operational reasons. Thus, there is a significant gap in our ability to measure and understand the contribution to production from individual stages, and this in turn makes it difficult to optimize the completion. Microseismic derived permeability provides an alternate answer in understanding the potential production contribution from each stage, with the added benefit that this is available immediately after completion of the hydraulic fracturing, without the need for well intervention.

Understanding zipper fracking

Over the past three years, zipper fracking has taken the unconventional shale market by storm. In comparison to completing an entire well at once, this method alternates the completion from well to well, one stage at a time, back and forth like a zipper down the length of the wellbore.

This method has become popular among operators because the logistics allow massive improvements in completions efficiency. However, it is important also to understand the effect on completion effectiveness and ultimately, production.

Most signs point to improved production from the zipper fracking method, but there are still questions on the treatment order: whether or not the outer wells should be treated before the inner wells.

In Figure 4, all three wells had a similar completion and treatment with the exception of fracking order. In completion A, the outer two wells were fracked before middle well, and in completion B, the middle well was fracked before the two outer wells.

To properly evaluate the impact of treatment order, it is necessary to look not only at the microseismic events, but also at the proppant-filled fracture volume achieved from each scenario.

As shown in the example, when the middle well is treated after treating the outer wells, the overall propped fractures are tightly constrained by the outer wells. The fracture growth is larger in the vertical direction compared to the middle well being treated first. In this case, the operator achieved sufficient perpendicular coverage, but too much vertical coverage. This suggests that in this area, treating the middle well first and using lower volumes of fluid will yield effective stimulation while also saving time and money.

Summary

In today’s commodity price driven environment, it is more important than ever for operators to focus on optimising production from each and every well. In order to continue to improve production, well completions must be compared, evaluated and improved upon over time.

Microseismic-based completion evaluation services can quantitatively and deterministically answer many of the questions operators are facing when trying to optimize production. This method of completion evaluation is the only available data-driven methodology to understand what is actually happening during hydraulic fracturing and to systematically improve production.

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