An often-overlooked operation in fibre-optic installations
If you’ve considered fibre-optics for your next completion, what can you expect to learn from it? Engineering and Geo-scientists look at data differently, FCS will discuss data observations from a Completion Engineer’s perspective.
We have an analogy that we mention to clients who are considering fibre-optics, “Fracing with fibre is equivalent to flying first class, you’ll never want to go back”. The data and learning’s from fibre are extensive. Every new job FCS takes on, we learn more that we can offer the next client. It always amazes us how each formation responds during stimulation – utilizing fibre allows you to look down the well and really understand what is going on.
In this article we will discuss the impacts of poor cement in a fibre-optic completion. Learning’s from fibre data can be applied to future completions, improving isolation and minimizing stage to stage communication.
Cementing Operations – The importance of a quality cement job, with proper casing/formation isolation in the lateral cannot be expressed enough. From hundreds of hours of data reviewed on various wells and formations, a critical and often overlooked aspect for optimal fibre data collection is cement.
FCS has been on projects where cement was poor, and others where the isolation was exceptional. What we are now able to quantify, is that ~ 55% of operators have poor cement jobs in the lateral section. What does this mean? Over-stimulation of an existing zone, cross-flow between stages, and risk to the fibre itself due to high velocities on un-protected fibre.
When a fibre operation is in the planning phases, we always recommend running a cement bond log. This allows us to determine where there is risk to the fibre, and when to cut a stage short due to over stimulation into an existing zone. The industry has moved away from cased hole logging on horizontals, but we always recommend this for any well with fibre installed.
A strong correlation to poor cement is deviation of the lateral. Undulating laterals due to poor well control are the most problematic. FCS has noted that in most cases where this problem was encountered, it was due to poor directional drilling, or the operator was attempting to follow the reservoir, as opposed to drilling a standard toe-up or toe-down configuration.
For a fibre completion, FCS recommends that the lateral be control drilled with dog leg severity kept under 4°. It is understood that this is a standard objective when drilling the lateral, yet a greater focus should be given when planning a fibre installation. Standoff of the casing should also be considered carefully, for optimal annular clearance we always recommend a minimum of two centralizers per joint, yet prefer three if allowable.
The cement slurry design is critical for zonal isolation in a fibre operation. We recommend considering a design that improves ductility and allows for expansion and contraction of the casing – minimizing cement failure due to stress.
Once the well has been cemented, it is recommended to have DTS on-line to observe the actual cure time of the cement. We do not recommend pressure testing the casing or opening of toe ports until DTS has confirmed the cement is of adequate compressive strength.
If a CBL is planned, the data may indicate poor to little bond across the proposed stimulated interval. In this instance, we recommend a pressure pass. In many cases, the bond improved significantly. With this data, it can then be assumed that there is zonal isolation when frac pressure is applied.
Once frac operations are underway, the DAS data may indicate cross-flow from poor cement bond. To help prevent further degradation of the cement, acid is dropped from the frac program. It was found to improve isolation of the stages uphole in some instances.
The example above shows what distributed acoustic sensing (DAS) can tell you in regards to behind casing communication. Zone B was the treated interval, yet flow was noted in previously stimulated intervals C and D. Zone A, which was yet to be perforated, recorded flow –due to poor cement. Flow into zones A, C and D were confirmed with distributed temperature sensing (DTS), which revealed cooling in all zones.
Below is an example of excellent zonal isolation, no flow was recorded above or below the treated zone. This was evident for the length of the lateral for all stages. The acoustic activity above the treated zone was attributed to cable vibration.
Other observations from cement quality include cluster efficiency. In areas with poor cement, channeling behind casing can occur. 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.
Where this becomes a concern with respect to fibre risk, is when abrasive flow moves across un-protected fibre. Depending on the type of fibre run, the cable will handle a certain degree of erosion, yet we have witnessed fibre failure up-hole where there are no perforations. The example below shows a failure due to erosion just outside the treated zone.
The advancement of fibre-optics in the energy sector has allowed operators to better understand the importance of a well designed completion. Cement integrity minimizes stage to stage communication, reduces over-stimulation and offers more opportunity to access reservoir in completions with tight spacing.
If you’d like to learn more about how to optimize your completion, contact Fibre Completion Services today.
