Operators across North America have recognized that groups of delineation (parent) and development (child) horizontal lateral wells often underperform when the delineation wellbores produce substantial volumes prior to stimulation and completion of the adjacent development wellbore(s). The relative degree of reduction in production per acre varies from one play or formation to another, but the condition is generally attributed to a phenomenon called well bashing.
Some operators will ascribe the cause to simple communication between stimulated reservoir volumes, but the concept of well bashing likely involves more than direct fluid flow between wellbores. Fracture mapping statistics gleaned from approximately 14 years of microseismic and microdeformation activities consistently indicate that induced fracture asymmetry in the direction of wellbores with lower pressure can result in productive rock volumes that are effectively bypassed when a development well is stimulated.
Figure 1 is a synthetic demonstration of several stages of microseismic mapping where asymmetric fracturing toward the AD (an area that has been drained by a given wellbore or wellbores) of the offset to the east has resulted in loss of production from the volume of reservoir that is not substantially stimulated (yellow area to the west).
Not only can reserves be bypassed, but the term well bashing originated when operators noticed that stimulating a new development well adjacent to an older producer often resulted in an immediate “kill” of the older delineation well. Sometimes the cessation of production was permanent, while other cases required quite a bit of operational effort to restore production to levels seen prior to the bashing incident.
The initial industry reaction to the well-bashing phenomenon was to blame it on inter-well communication and focus on methods that might prevent that communication, such as adjustments in parallel lateral spacing and/or decreasing stimulation volumes pumped per unit of lateral length. Unfortunately, these approaches may have been flawed, in that:
- Under some circumstances, inter-well communication can be beneficial from a recovery factor perspective, as it can assist in accessing common reserves that are proximate to a particular wellbore, but not connected to it. When this happens, the second wellbore can produce those reserves, if there is sufficient communication between the two respective fracture networks.
- Reducing the stimulation volumes per unit of lateral length has an undesirable side effect of reducing the total exposed induced fracture surface area, which can negatively impact the rates and volumes of reserves recovered.
- Increasing parallel lateral spacing to the point that there is little or no communication between conductive fracture networks can leave a strip of unstimulated or understimulated reservoir volume between wells.
Evidence of Asymmetric Fracturing
Examination of amalgamated microseismic records over a 14-year period, along with the application of rigorous geomechanics and public production statistics, has provided confirmation of a solid relationship between reduced proximate reservoir pressure and asymmetric induced fracturing trending toward the rock volume with the lower pressure. More recently, 3-D numerical unstructured grid reservoir modeling has assisted in the quantification of the phenomenon.
The static pressure gradient, reservoir system permeability and the degree of permeability contrast between reservoir layers appear to control the degree to which asymmetric fracturing can be an issue. The degree of asymmetry, in turn, impacts the extent to which the long-term time between completions affects recovery of hydrocarbons over and above simple volumetric depletion.
North American far-field fracture mapping data archives suggest that some plays typically experience asymmetric fracturing in the direction of lower-pressure AD to a higher degree than other plays. The Bakken/Three Forks and the oil window of the Eagle Ford Shale are two examples where induced fracture asymmetry can be so extreme that fractures can cross two, three or even four proximate parallel wellbores on their path to a lower pressure (and therefore lower stress) zone. In both of these plays, the bashing can be so severe that production from the parent delineation wellbore is permanently lost if measures to mitigate the problem are not implemented.
Other plays can experience the asymmetry to a substantially lesser degree, and sometimes the parent well that has temporarily experienced a kill is easily brought back online. Often in this scenario, the parent well subsequently experiences a bump in productivity over and above simple transient flow, whereby it is assumed that some of the frac fluid and proppant from the development well assisted in re-stimulating new rock in the old delineation well drainage volume.
Extreme asymmetric fracturing can materially impact reserves and rate of recovery if the acreage position of a given project is substantial. Numerical modeling suggests that, depending on the severity of asymmetry and how many wells per pad are impacted, the bypassed reserves can impact 200 million to 800 million cubic feet of reservoir per incident. Depending on the commodity price environment, for a typical 200,000-acre development, this could translate to a billion-dollar issue.
The industry appears to be progressing toward two methods to mitigate the negative impact of asymmetric fracturing and well bashing:
- Shortening the long-term time between completions. This process can be capital-intensive and carries with it incremental risk of bypassing a rigorous engineered learning curve. Fundamentally, it involves initiating offset development drilling as quickly as possible after the delineation wellbore clearly demonstrates commerciality. This approach helps ensure that the total volume of hydrocarbons and water extracted by the delineation well is small, and therefore the pressure decline in the delineation well’s drained volume is also small. Unfortunately, in a severely distressed commodity-price environment, development well drilling is often difficult to justify, unless holding acreage or satisfying previously negotiated drilling commitments is in play.
- Performing protective refracturing operations on the delineation wellbore. Over the last two years, this process has demonstrated promise as an effective and economic solution to the well-bashing phenomenon.
If an unconventional ultra-low-permeability well has produced a certain volume of hydrocarbons and water over some particular timeframe, it can be assumed that there is a proportional decrease in the general pressure of reservoir rock near the induced fractures, since volumetric depletion is the primary driver of production when permeability is extremely low. One of the methods to temporarily arrest this decline in pressure is to place large volumes of water-based fluids back into the rock volume where pressure depletion has occurred.
Protective refracturing is a controlled process whereby fluids are pumped to first fill existing propped and drained voids, then subsequently break and stimulate new rock in the lower-pressure environment.
The result is that the stimulated volume of reservoir surrounding the induced fractures generally increases in pressure, which can lead to the reduction or prevention of severe asymmetric fracturing from an offset development well.
A well-designed protective refrac fundamentally addresses two scenarios. First, it attempts to mitigate the negative impact of the offset asymmetric fracturing (and associated loss of reserves) as previously discussed. Second, the refracturing treatment stimulates new reservoir rock volume in an effort aimed both at accelerating the rate of reserve recovery and increasing the estimated ultimate recovery of the original delineation well. This is accomplished with a well-designed and executed process involving an upfront pressurization process, custom diverter application, and propped fracturing with careful control of cycle timing and volumes.
Protective refracturing is well-suited to challenging commodity price environments, as it is one of the lowest cost per barrel of oil equivalent processes in a larger portfolio of delineation and development well drilling.