No abstract

One of the critical factors in planning a hydraulic fracturing treatment is proppant selection. Conductivity is an important property for ensuring efficient reservoir stimulation. Conductivity data obtained during laboratory measurements help determine which proppant to use. ISO provides standard testing procedures and methodology for measuring the long-term conductivity of proppants but does not document expected measuring uncertainty. This paper addresses accuracy and uncertainty estimation of ISO 13503-5 conductivity measurements. For the purposes of this study, three proppant samples were used. Two identical samples of high-strength ceramic proppant (HSP) and one sample of natural silica sand were distributed among six laboratories that participated in this study. The resulting data from measuring conductivity and permeability of the samples were used to calculate repeatability and reproducibility in terms of standard deviations in accordance with ISO 5725-2. Uncertainty of measuring the ISO long-term conductivity of proppants was calculated in accordance with ISO 21748. The evaluation shows that conductivity and permeability values of ceramic proppant and natural sand measured by the ISO 13503-5 method have quite high interlaboratory uncertainty. Uncertainty of conductivity is from 19 to 100% depending on closure stress for values of conductivity from 3000 to 200 md-ft respectively. Uncertainty of long-term conductivity measurements increases dramatically while conductivity decreases. At the same time, internal evaluation of uncertainty in measuring conductivity within one laboratory resulted in approximately 15% as reference value of uncertainty for a single laboratory and never exceeded 30% in given conductivity range. ISO 13503-5 prescribes testing procedures; however, it does not provide information about measurement uncertainty. This information is essential for comparing different proppants to enable reasonable selection criteria for a job, to identify proppant property inconsistency and quality decline over time due to changes in production process or raw material, or to resolve discrepancies in different laboratory measurements. Uncertainty values were estimated for a wide range of conductivity data and different proppant types.

Guar-based crosslinked fluids remained the prevalent choice of frac fluid for a long period of time, since massive hydraulic fracturing was started in Russia. Traditional frac fluid contains 2535 ppt of crosslinked guar, which results in very high fluid viscosity (min 400 cp at 100 sec-1 as rule of thumb) and low retained permeability of proppant pack - around 35%. With recent move towards complex geology reservoirs in Russia, where wide propped frac is no longer an optimum solution, the need in review of current fracturing approaches emerged. In several last years local operators started to gradually move away from h igh-viscosity fluids via its partial replacement with cleaner guar-based low viscous linear gel. However, even in this case retained fracture conductivities are typically not higher than 60-70%, especially in cases when hybrid fluid systems are used - linear fluid combined with crosslinked gel. Goal to reach improved fracture conductivity opens a field for new discoveries. This study objective is to evaluate the applicability of novel clean frac fluid for conventional reservoirs in Russia. Current study is focused on development of laboratory testing procedures and testing results analysis of novel synthetic polymer-based fracturing fluid in terms of its applicability on conventional reservoirs - tight sandstones. Viscous slickwater has already been widely used on shale reservoirs in North America, however was never applied for conditions of sandstones fracturing: in mili Darcy environment, in combination with ceramic proppant, pumping via tubing, utilizing pump rates less than 10 m3/min (60 bbl/min). Fluid rheology studies, leak-off behavior, regained conductivity of the proppant pack, regained permeability of the formation, dynamic proppant transport tests and dynamic fluid viscosity evaluation are described in the paper. Elastic properties of viscous slickwater (H.Zhao, S.Danican, H.Torres, Y.Christianti, M.Nikolaev, S.Makarychev-Mikhailov, A.Bonnell, Schlumberger, 2018) provide improved dynamic proppant transport and static proppant settling, in comparison with low viscous fluid - linear guar-based gel, i.e. better horizontal and vertical proppant distribution inside the fracture. Ceramic proppant pack conductivity even with high loadings of High Viscosity Friction Reducer without breakers showed superior results - Regained conductivity reached 100%. Coreflow experiments using conventional (1-10 mD) sandstone cores demonstrated 100% regained phase permeability to hydrocarbon, proving that fluid is non-damaging to formation. As a result of numerous laboratory studies performed, Viscous slickwater was qualified as alternative fracturing fluid to conventionally used guar-based gel and has been approved for field testing campaign on conventional tight sandstones in Russia. Field trials of novel frac fluid - Viscous slickwater demonstrated positive results both in terms of pumpability and well productivity on tight sandstones 0.5-3.0 mD This fluid has been recommended for further roll out to wider range of conventional oilfields.