Utilizing fibre-optics to improve on current completion methods.
Part 3 of our series on the measurable benefits gained from deploying fibre-optics. In this article we will be discussing cluster efficiency. This will be a general overview, allowing the reader to understand how utilizing DAS will assist in optimization of the completion.
A cluster can generally be defined as a set number of perforations, shot into the casing and repeated over a number of intervals. The cluster groupings then form the stage that will be stimulated. A stage may have as little as 1 cluster, to more than 15. The design of the stage and number of clusters are dependent on many variables. Below is an example of a typical stage, profiled as a casing cutaway.
Cluster efficiency forms a critical aspect to the optimization of a completion. Current industry rule of thumb for cluster efficiency sits at ~ 70%, meaning that only 2/3 of the clusters are active. In the example above, only ~ 3 clusters would be open for stimulation.
What are some of the causes of poor efficiency (there are many, below are most common)
- Limited entry design
- Reservoir heterogeneity
- Poor cement bond
- Acid placement for breakdown
- Diverter use
- Cyclic pump rates – causing cluster shutdown
Limited Entry Design
The majority of operators utilizing plug and perf completions have shifted to limited entry design. Limited entry is a form of perforation design, limiting fluid injection to a small number of perforations. Fluid distribution is considered to be more uniform across clusters due to high perforation friction.
Perf efficiency can be affected by incorrectly calculating the required differential pressure needed to open all clusters. Many wells experience heel bias flow, and without a high-pressure differential, toe end clusters will not open.
An item to consider when planning a limited entry design is velocity and erosional effects around the fibre cable. The fewer the perforations per cluster, the higher the velocity. It is not uncommon (in extreme limited entry) for perforations to erode from the standard 0.37” OD to more than 2.00” OD. Erosional impacts were verified through downhole camera work on a number of completions.
FCS has observed a number of limited entry designs with various operators, with as little as 300psi differential, up to 3,400 psi differential. When extreme limited entry is planned, erosional risk needs to be reviewed. Two different operators chose a single shot cluster design, with 4 clusters per stage. The risk FCS noted was when a cluster sanded off, which resulted in extremely high velocities. The result was cutting the frac was cut short due to the erosional impact on the fibre.
Reservoir Heterogeneity
Cluster efficiency can be directly affected by the location of the cluster along the lateral. The majority of operators place stages in a geometric pattern, which are evenly spaced along the lateral – regardless of the lithology. Efficiency can be affected by laminated/inter-bedded layers within the reservoir, which may have higher breakdown pressures.
If a cluster is placed within a higher-stress region of the reservoir, they will not breakdown at the same point as clusters within a normally stressed section of the reservoir.
In areas of high heterogeneity, some operators are now using the drilling data to assist with stage placement for improved stimulation effectiveness. In the example below, a geo-mechanical completion is shown. The stages were moved into the lower stress regions of the reservoir, with varied spacing between stages.
Cluster efficiency improved 40% on average within these reservoirs, with consistent breakdowns across all clusters within the stage.
Cement Bond
Cement bond on a completion is one of the most critical aspects for stimulation effectiveness. FCS has previously written a paper on the importance of cementing, with the most commonly observed issue being annular flow behind casing.
In operations with either limited entry or conventional perforating, flow distribution was poor due to communication. With little cement behind casing, the completion essentially becomes an open-hole completion.
Flow through the perforations will move toward the most dominant cluster within the stage.
Hours can be spent engineering the right limited entry design, yet without a good cement bond, cluster efficiency will be low. FCS has observed cluster efficiencies < 40% on wells where extreme limited entry was used. The reason for poor efficiency was flow behind casing, starting at the heel end of the cluster.
Diverter
There are two schools of thought with respect to diverter, it either works, or it doesn’t. What FCS has observed through many diverter trials, is that diverter is not used effectively in the majority of cases. The most common observations from fibre data when diverter was deployed:
Erosion:
- Erosion of the perforations is not taken into account when deploying diverter. Mechanical diverter comes in specific sizes, and selected based on the un-eroded perforation OD.
- A common method of diversion is to divert after a certain tonnage has been pumped, in most cases 50% or more of the frac.
- In extreme limited entry (XLE), there are normally 6 or less perforations per cluster. Erosion of these perforations will be significant, especially in the event of poor cluster efficiency.
- Fibre data has confirmed that in most instances, the diverter only worked for a short period due to the diverter passing through the eroded perforations, which were larger than the diverter product.
- To mitigate this issue, diverter was then pumped earlier in the frac before erosion became a concern. Where this methodology fails, is that when pumped early – the operator is effectively shutting down a cluster before an effective stimulation could be placed.
Pressure:
- If mechanical diverter is used, most service companies will recommend a 30% diversion.
- Where this becomes a risk is when there is poor efficiency in an XLE design.
- Diverter can effectively shutdown the stimulation when an operator follows the diverter program, as opposed to actual on-site observations from the fibre data.
- As an example from a past operation, the operator dropped 5 pods in a 6 cluster stage, with 3 perfs per cluster. At the time, there were only two active clusters. Once diverter was dropped, a significant pressure increase was noted, due to 5/6 perfs being shutdown – with no dormant clusters opening.
Flow Re-Distribution:
- In most instances, FCS has noted that diverter did not open dormant clusters – which was the intent with diversion.
- What occurred was a shutdown of poorly producing heel end clusters, which “re-distributed” the flow to an already dominant cluster. Clusters that were dormant did not open – which we believe is due to acid placement and poor to little breakdown of the dormant clusters.
Deployment Method:
- In all cases with diverter deployment, sand was cut to drop pods.
- Once pods were in the wellbore, rate was dropped in order to reduce pressure prior to the pods hitting the perforations.
- Where this becomes ineffective is that many reservoirs are sensitive to rate. Once rate is dropped, clusters close off and do not re-open again. This is where diverter can further compound poor cluster efficiency, with clusters shutting down, there is an excess of pods to fewer perfs – with a poorer result once the stimulation resumes.
Acid Placement
- Not all completions require acid for perforation breakdown. Operators routinely run acid to assist with breakdown due to the cement.
- In reservoirs that do require acid due to high pressure, it is critical that the acid be pumped in the wellbore such that it is placed evenly across all perforations.
- A risk that has been observed in the field is poor acid pumping tie-in and set-up. To ensure the most effective acid placement, the acid needs to be directly behind the ball in a plug and perf operation.
- Some operators tie acid in directly at the manifold, with the ball being dropped directly at the wellhead. What occurs is the ball seating in the plug, with acid several m3/bbls above the bridgeplug. The result is a bullhead squeeze of the acid, which normally only goes into the heel end clusters.
Pump Rates
- As mentioned above, reservoirs can be very sensitive to pump rates.
- Once clusters breakdown and open to flow, it is not recommended to lower pump rate, unless there is a velocity/erosional risk due to dominant cluster flow.
- In the example below, pump rates were lowered and 3 clusters shut off. When pump rates increased, the clusters remained shut down
Summary
There are many learning’s from fibre data that can be applied to conventional completions without fibre-optics. An operator can learn how their particular formation responds to stimulation and the overall effectiveness of the engineered design with fibre.
Maintaining cluster efficiency is one element of optimizing a completion – with the right engineered solution. Fibre-optics makes this possible, applying the learning’s to future field development.
If you’d like to learn more about how to optimize your completion, contact Fibre Completion Services today.
