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Deliverables
Numerical modelling was conducted by using recently developed software called �WellWhizTM� from Fracture Technologies Ltd in the UK. WellWhizTM is essentially a black oil reservoir simulator, which is capable of modelling the pressure drop in the reservoir and along the fracture numerically using the non-Darcy equation: dp/dl = (uv/k) + (bp/v²). In this respect, the software is unique in simulating the drag forces (psi/ft) along the fracture, from which it possible to predict the tendency of proppant flow back in the well.
A one well model was built (using WellWhiz software) with a hypothetical drainage area as shown in Fig.-11. Two proppant sizes were modelled, namely 20/40 (historically used in the North Sea) and 12/18 Mesh (used in the Clyde application well).
Three cases were modelled as follows:
-12/18 proppant with a concentration of 2 Lb/ft2 to model the fracture treatment implemented by Clyde Petroleum.
- 20/40 proppant with a concentration of 0.75 Lb/ft2, to model a stable fracture using small proppant, which has been historically been used and is still being used in the North Sea.
- 20/40 proppant with a concentration of 2.0 Lb/ft2, to model a similar fracture width as that for 12/18. It should be noted that 20/40 proppant at 2.0 Lb/ft2 would not be stable as indicated from the laboratory test discussed earlier in this report.
Based on the laboratory test results, it was decided to consider a proppant fracture treatment utilising 12/18 Carboprop at a concentration of 2 Lb/ft2, as presented in Form-A. A larger proppant (8/14 Mesh) size was not considered due to the following,
- 8/14 Mesh is not a standard size and hence the lead-time to order such material would be in excess of 4 months.
- Furthermore, 8/14 is considerably larger than 12/18 and would hence carry an extra risk of placing this material successfully in a fracture.
Clyde Petroleum selected a suitable well for implementing this technology offshore the Netherlands. The well was drilled March 2001 and tested at 650,000 N.m3/Day at a BHFP of 90 Barg (reservoir pressure of Ca. 260 Barg). The results were disappointing and hence it was decided to fracture the well to enhance the production rate.
A fracture treatment was designed with a half-length of 200m and a proppant concentration of 2 Lb/ft2 using 12/18 Carboprop. The proppant was resin coated for added insurance; nevertheless the design honoured the stability criteria. The fracture treatment was implemented in May 2001, and the well has been on production since.
The fracture geometry with a fracture half-length of Ca. 200m. Finally the proppant concentration indicates an average value of 1.80 Lb/ft2, which is in reasonable agreement with the intended design of 2.0 Lb/ft2.
The post fracture results indicate a production rate of 1,250,000 N.m3/D of gas at an initial bottom hole pressure of 150 Barg. This essentially suggests a 3.50 folds increase, which is slightly higher than expected.
The production from this well was routed through a proppant-filter unit especially built for this project. Periodic inspection of the filter unit has indicated ZERO proppant flow back to date. Clyde considers this a successful out come, in particular, as the well is located on un-manned platform. Based on these results it is now possible for Operators to consider fracturing sub sea wells to exploit marginal gas field developments in the North Sea.
Mechanical control of proppant flow back has in the past been limited to ¿sand control¿ type of completions where screens and gravel packs have in certain circumstance been used to stop proppant flow back. Such completions recently termed in the industry as Frac & Pack have been implemented primarily in the Gulf of Mexico in soft sand reservoirs. Published work indicates that such completions yield relatively high skins between (-2 up to +5). This would be considered a poor result when applied to gas basins in Europe where a successful fracture treatment would yield skins between ¿5 and ¿6.
Hence, conventional ¿frac & Pack¿ completions were not considered in this study.
In stead a brief review of the market suggests that mechanical tools for the purposes of limiting post fracture proppant flow back in hard sandstone formations do not exist. Halliburton Services have suggested that such a tool could be developed if sufficient funds were made available. Baker Hughes do however have a tool ¿perf valve¿ which may be adapted for this application. ¿Perf Valve¿ is relatively a new tool on the market and consists of short sub (2-3 ft) in length, which would be placed in the liner at an appropriate interval, with a set of perforations, which may be shut off with a sleeve. The tool may be adapted with a shut off sleeve with a certain mesh size to limit proppant flow back.
The advantage of the tool is that it saves the operator time and cost by eliminating the need to perforate. The well is then hydraulically fractured fracturing and placed on production. If proppant flow back is problematic, the sleeve is shifted.
The disadvantage of the tool is that the sleeve is short (2-3 foot) and hence is only suitable in horizontal wells where multi-fracture treatments are required, where the fracture azimuth is perpendicular to the well trajectory. The tool can not be used in vertical wells where the fracture height is in line with the well trajectory and would be in 10¿s of metres. In such a case it would be advantageous to maximise the contact between the well and fracture height, as in the application treatment conducted by Clyde offshore The Netherlands. Furthermore, proppant movement in the fracture might result in plugging the mesh sleeve in the tool resulting in a skin factor at the well bore which would reduce the well deliverability.
Furthermore, the proposed tool modifications (where ¿perf valve¿ is equipped with a mesh screen to limit proppant flow back was not actually attempted or tested). It is recommended that erosive effects of proppant flow against the mesh sleeve should be investigated during future development work.
Coil tubing is required to shift the ¿Perf Valve¿ sleeve. Hence the platform should be designed with a crane capable of lifting such equipment. The special crane requirement and coil tubing operations will add considerable cost to the development and such operators will increase the risk of the production loss.
In summary, at the time of the study mechanical tools to limit proppant flow back in a vertical well do not seem to exist, such tools should be considered in future development work.
In order to gain an understanding of how the size of the proppant controls the proppant pack stability, tests have been run using four standard sizes of un-coated proppant. These tests have been performed simulating different conditions relevant to the production rate of gas wells in Europe. In addition, two technologies developed to overcome proppant flow back have been included in the programme. These are resin coated proppant (RCP) and Schlumberger�s fibre technology.The laboratory tests were carried out with the following consideration:
-Type of proppant, e.g. regular (un-coated) resin coated, fiber technology, etc
-Proppant size in relation to the fracture width.
-Drag forces along the fracture, which are related to flow-rate and fluid phase saturation.
-Test temperature, duration and stress cycling levels.
The results of the tests indicate that non-resin coated proppant would be stable at fracture widths to proppant diameter ratio of 5 to 1.
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