The Vincent reservoir was drilled with an 8-1/2” openhole using a Water Based Mud (WBM) drill-in fluid selected in a two stage testing program (Table 1). The initial static filtration tests used 12 and 39micron aloxite disks to test eight WBM of different particle size distribution (PSD). The results showed no major difference between the various WBM indicating the formation was resilient to damage over a range of PSD (Fig. 1). The final return permeability tests were conducted for two muds using synthetic core, the results showed an acceptable 70% return permeability for the mud selected for Vincent.
Table 1. Vincent WBM and SFWBM Composition and Properties.
|Additive||WBM Concentration||SFWBM Concentration|
|Water – Base fluid||0.8515 bbl/bbl||0.898 bbl/bbl|
|Potassium Chloride – Density and shale inhibition||40 ppb||42 ppb|
|Sodium Chloride – Density and shale inhibition||0||42 ppb|
|Xanthan Gum – Viscosifier||1 ppb||0 ppb|
|Starch – Fluid loss control||6 ppb||6 ppb|
|Shale inhibitor||12 ppb||12 ppb|
|Magnesium Oxide – pH stabilizer||2 ppb||2 ppb|
|Sized Calcium Carbonate – Density control and filter cake building||46 ppb||0|
|Biocide||0.25 ppb||0.25 ppb|
|De-mulsifier||1 ppb||1 ppb|
|Property||WBM Properties||SFWBM Properties|
The Vincent WBM was not designed to pass through the sand screens. If this WBM was flowed back through the sand screens with no conditioning, it would result in sand screen plugging and lower well productivity.
Sand Screen Selection
Sand retention slurry tests carried out using Vincent core samples showed sand screen sizes in the range of 200 to 300micron were suitable. The next step was to test sand screens for mud plugging capacity. A mud with 60ppb calcium carbonate (CaCO3) loading was pumped against ten different premium sand screen disks of 200 to 310 micron aperture size. The filtrate volume passing through the sand screen samples at 100psi pressure differential was used for comparison. The filtrate volume passing through the sand screen before plugging occurred varied from 97 to 112ml, a relatively low 15% difference in mud plugging capacity from the worst to the best (Table 2). This data shows if the WBM PSD is sufficient to plug the sand screen with the largest aperture, then plugging performance is not a significant differentiator between these sand screens. The shape of the sand screen plugging curve has a steady increase followed by rapid deterioration showing the sand screen can tolerate a high level of plugging before production would be severely impacted (Fig. 2).
Table 2: Sand Screen Mud Plugging Test Results.
|Worst 97ml||230||Twill Weave|
Although other factors were involved the sand screen selected for Vincent had the highest plugging capacity, a square weave, a porosity of 60% and an average aperture of 263micron measured by glass bead tests.
Sand screen plugging risk was reduced by maximizing the filter media length and area on each 5-1/2” tubing joint to 8.26m and 4.05m2 respectively.
The first step was to decide which fluid would be left in the cased and openhole sections of the well prior to installing sand screens. This was a balance between the risk of openhole collapse due to losses, versus lower well productivity due to sand screen plugging. Openhole collapse is an increased risk for Vincent wells as the lateral needs to remain stable with the fluid for a period of 3 to 7 days. To assess these competing objectives brine, Solids Free Water Based Mud (SFWBM) and WBM were considered (Table 3).
Table 3: Displacement Fluid Options Comparison.
|Brine||Solids Free Water Based Mud||Water Based Mud|
|Hole collapse risk||Highest||Medium||Lowest|
|Sand screen plugging risk||Lowest||Medium||Highest|
The SFWBM was selected as a compromise between the two risks. The starch in the SFWBM has filtercake building characteristics, the rheology is low to improve WBM displacement and SFWBM can flow through sand screens without plugging (Table 1). Both the openhole and cased hole were displaced to SFWBM before installing sand screens.
After displacing the WBM used in drilling the lateral to SFWBM, the fluid in the openhole would be a mix of SFWBM and any remaining WBM. The openhole fluid would therefore contain solids at a low concentration to further assist in maintaining the filtercake.
Openhole Displacement Volumes
The objective of the SFWBM displacement process was to remove as much WBM from the well as possible. The SFWBM had a density matched to the WBM to improve WBM displacement, a lower density SFWBM could over-run the WBM leaving more WBM behind. A flow rate limit through a rotary steerable tool in the drilling bottom hole assembly (BHA) imposed a maximum pump rate of 650gpm. Cement displacement software was used to give an indication of the optimum WBM displacement methodology, the key findings from this analysis were:
- The openhole remained within an acceptable equivalent circulating density at 650gpm pump rate.
- WBM removal efficiency plateaued at SFWBM volumes in excess of 2.5 x openhole volume for both 1000 and 2000m openhole lengths.
A further consideration was the concern of potential filtercake erosion with excessive circulation of SFWBM could lead to losses and possible lateral collapse. As minimizing sand screen plugging was a key objective it was decided to displace the openhole with 2.5 openhole volumes and if needed optimize the volume based on field results.
WBM Conditioning Prior to SFWBM Displacement
With productivity being such a key driver every effort was made to maximize fluid cleanliness and clean up efficiency at each stage of the completion process. On this basis it was elected to not only focus attention on the SFWBM aspects, but also to devote significant effort in ensuring optimum conditioning of the openhole WBM left in hole after drilling.
Although capable of generating comparative results, the cement displacement software was not ideal for modeling the complex displacement process of different fluid rheologies with pipe rotation. The accuracy of the model for determining how much WBM would remain in the openhole was uncertain and as a consequence the risk of sand screen plugging with residual WBM could not be eliminated. To mitigate this risk the WBM in the well was conditioned through 77 and 66 micron shaker screens prior to SFWBM displacement. These fine shaker screens were installed in the later stages of drilling each lateral (final ~200m) in order to start the clean up process as soon as possible. The WBM was conditioned when header box samples passed a Flow Through Test (FTT) using sand screen coupons three consecutive times.
The FTT apparatus used a 1.9” sand screen sample, 1litre WBM sample and a 10psi pressure differential. The time for each 0.2litres of WBM to pass the screen was recorded. Failed FTT tests were when the sand screen sample plugs before 1litre has passed, or if the time increments increase for each subsequent 0.2litres indicating plugging. A FTT pass is when the time increment between each subsequent 0.2litres remains constant (Table 4).
Table 4: Examples of Flow Through Test Fail and Pass Results.
|Incremental Time (Seconds)||FTT Fail||FTT Fail||FTT Pass|
|0.4litres||Plug @ 0.23litres||3||3|
The WBM conditioning and FTT analysis is on the rig critical path, so it is inevitable the WBM is conditioned to the point of just passing the FTT. Provided the WBM volume that could pass through the sand screen when it is installed is less than 550 litres/m2 (equivalent to 1litre passing through a 1.9” sand screen sample), then the FTT is a valid sand screen plugging
A sand screen plugging risk not addressed is the filtercake consists of unconditioned WBM solids. This may be answered by comparing the amount of WBM solids needed to plug the sand screen, with the solids available in the filtercake. The unconditioned WBM typically plugs the FTT coupon after 0.2litres of filtrate has passed, this is a filter mesh plugging capacity of 18kg/m2. The available solids in a 2mm thick filtercake is 12kg/m2, so the sand screens should not totally plug.
Initially new WBM was used for each lateral to keep it within specification and limit the impact of drilled in solids on the WBM PSD. During the well program it was apparent the WBM was in good condition at the end of some laterals. Instead of replacing the mud it was re-used provided it met the following criteria:
- Methylene Blue Test < 2.5ppb (and preferably as low as possible).
- API fluid loss is < 4mL.
- PSD looks steady based on continuous readings during the drilling phase.
Openhole Clean Up Summary
The openhole clean up practices put in place for Vincent can be summarized as follows:
- Renew or use clean WBM for each lateral: Keeps the WBM in specification.
- Condition the WBM to meet FTT: Ensure WBM left in the well is unlikely to plug the sand screens.
- Displace the openhole to SFWBM: Remove as much WBM as possible to minimize sand screen plugging.
- Displace the cased hole to SFWBM: Install sand screens in a solids free fluid to minimize sand screen plugging.