Fueled by the shale revolution, U.S. crude oil production from 2008 to 2023 increased from 5 million barrels per day to 12.9 million barrels per day, and annual natural gas production also rose from 25.6 trillion cubic feet to 45.6 trillion cubic feet. The shale revolution was made possible by horizontal drilling, hydraulic fracture stimulation and seismic imaging and monitoring.
But, unlike conventional high porosity and permeability reservoirs, tight shale wells experience rapid production declines, and some of the early wells were stimulated by less-than-optimal technologies. After some years, operating companies must restimulate the parent or child (infill) wells.
Shale restimulation in the U.S. plays is especially necessary for wells drilled between 2008 and 2016. It involves two important tasks: Selecting candidate wells and adopting an optimal refracturing method.
Candidate Well Selection
There is no single protocol for selecting candidate wells for refracturing, partly because each well has its own unique conditions and partly because the industry has yet to assemble a large database to help streamline this task.
Some of the criteria to be considered in selecting wells for restimulation are:
- Production criteria: Production performance, declining rate and estimated ultimate recovery of the well
- Spatial criteria: Lateral well length, original frac cluster spacing and offset distance between parent and child wells
- Original completion criteria: Drainage efficiency, failed fracturing stages and fracturing fluid and proppant intensity
- Subsurface geomechanics: Magnitude and (re)orientation of stress axes acting upon the rock formation, pore fluid pressure changes and rock matrix and fracture properties
- Age: Wells older than eight, six, or four years – depending on the operator’s financial strategy
At present, well selection is mainly based on field experience and numerical simulation; however, with increasing data, AI methods such as machine learning, expert systems and deep learning will be routinely used. Even then, each basin will have its own decision-making protocol.
Stress Reorientation
Hydraulic fractures usually develop in the direction of maximum horizontal stress. Prolonged oil and gas extraction from the reservoir increases the effective stress, and hence, fracture closure pressure. Studies have also shown that local stresses in producing shale reservoirs undergo significant changes in orientation, and even reversals within a year, due to pre-existing natural fractures and faults, shale bedding and long-term oil and gas production.
Two numerical simulations of stress rotations in shale plays, one by Zeng and colleagues published in Processes 12, and the other by Wang and colleagues published in KeAi Publishing’s Petroleum Science journal, reveal interesting insights:
- Extreme changes in stresses along x and y directions occur at different positions and different times, mainly controlled by pore pressure gradient. The greater the pressure gradient between the fracture and rock matrix, the greater the stress change.
- The larger the initial difference between the maximum and minimum horizontal stresses, the more difficult it is for stress reversal to occur. The stress reversal may not ever occur if the initial stress difference exceeds a certain limit.
- As the number of natural fractures increases, the rotation of maximum horizontal axis also increases. And as the approaching angle of pre-existing fracture increases (relative to the horizontal axis), the stress reversal area becomes smaller.
- Optimal time for refracturing is to avoid when the maximum horizontal stress rotation is the largest (that is 0 to 20 degrees with respect to the horizontal x axis), so that the new fractures develop perpendicular to the horizontal well.
Shale Restimulation Methods
After the well is selected for refracturing, it is cleaned out. There are two basic approaches to refracturing: “temporary plugging and diverting refracturing” (TPD) and “wellbore reconstruction refracturing” (WR), also called “mechanically isolated (ISO) refracturing.” Let’s take a look at how these break down.
- TPD: This involves initially plugging the perforated zones by injecting a temporary plugging agent, then diverting the fluid flow to create new fractures in the rock formation sequentially, according to the formation’s opening pressure sequence. TPD is usually used to refracture the fracture clusters which were not adequately stimulated. TPD is less expensive but also less efficient, as plugging previous fracture clusters poses a challenge.
- Nested-casing Refracturing: This WR (or ISO) method involves placing a smaller-diameter casing string within the original wellbore, creating a “new well” within the old well. Then the same or new perforation points are refractured. In the United States, the common designs are a 4-inch casing inside of a 5.5-inch casing or a 3.5-inch casing inside of a 4.5-inch casing. This method is mainly used for wells experiencing huge production declines. A study of the Fuling gas field, China’s largest shale gas field, by Shi and colleagues, reported in Science of the Total Environment, shows that ISO methods are more efficient but more costly and consume more water than the initial fracturing.
- Expandable Liner Refracturing: A type of ISO refracturing in which an expandable liner is inserted into the wellbore and is inflated by high-pressure fluid pumping. Then, old perforation clusters (production zone) are isolated laterally, and new perforation clusters are created in the wellbore.
Refracturing is not new: In the past, it has been applied to coal-bed methane and tight gas sandstone formations. But shale plays are different, and shale restimulation is a new revolution. Nevertheless, technically and economically successful shale restimulation requires high-resolution geoscience and smart engineering.