Fracture Growth Monitoring in Polymer Injectors- Field Examples

Abstract

The objective of injecting polymer in brown fields is to increase recovery beyond primary and secondary recovery mechanisms. However, generally it is difficult to achieve adequate (viscous) polymer injectivity in depleted sandstone reservoirs without fracturing. Therefore, monitoring fracture propagation is required in order to control vertical conformance and areal sweep and avoid early polymer breakthrough. Different surveillance methods are used to identify the existence and properties of fractures in polymer injectors. Pressure Fall off (PFO) survey data in conjunction with time-lapse temperature surveys are extensively used to determine the fracture dimensions. PFO tests in Polymer injectors have particular characteristics since they are influenced by shear-dependent viscosity seen in non-Newtonian fluids. A specially developed Injection Fall-off (IFO) model was used to determine fracture dimensions which is based on exact semi-analytical solution to the fully transient elliptical fluid flow equation around a closing dynamic fracture developed by Shell, (Van den Hoek 2005), as static fracture models are inadequate. This paper presents different phenomena in polymer injection seen in PFO tests such as fracture closure, the effect in-situ polymer rheology and the detection of the polymer front. The paper demonstrates the effect of liquid-level drop observed in PFO survey in under-pressured reservoirs and its impact on determining fracture and some other reservoir properties. It also shows how plot-overlays of time lapse PFO's for a particular well can be used to track changes in fracture dimensions. All of these are illustrated by a number of field examples of polymer PFO which also demonstrate the calculated fracture dimensions from the data. Finally, some recommended best practices are suggested for fracture monitoring. IntroductionThe large sandstone brown field that the polymer injection is taking place in is located in the eastern side of the South Oman basin. The oil is heavy, 22 API and viscous, 90 cP. The field is highly heterogeneous with sand, diamictite and shale bodies. Nonetheless in the main reservoir units the Net Sand to Gross reservoir ratio approaches one and the permeability can range up many Darcies. The main objective of injecting polymer is to increase the recovery beyond the primary and secondary recovery mechanisms by improving the sweep efficiency (figure1). In such heterogeneous reservoir, it is difficult to achieve an adequate viscous fluid injection under matrix condition in depleted sandstone reservoir. Therefore, Polymer injection was designed to be injected under controlled fracture conditions where the fracture length should not exceed 1/3 of the distance between injector and producers over the life of the project. This requires intensive qualitative and quantitative monitoring of the fracture dimensions through different surveillance techniques to control the vertical conformance and avoid early breakthrough of the polymer.

Alkali Polymer Flooding of a Romanian Field Containing Viscous Reactive Oil

“…The same polymer at identical concentrations as those mentioned here is presently being injected in the Romanian field. The significant pressure difference observed in the core flood experiments suggests that induced fractures may be created under field settings, as documented in other studies (e.g., Shuaili et al [36], Moe Soe Let et al [37], and Zechner et al [38]). Field applications of polymers have demonstrated that a significant portion of increased oil production is due to flow diversion rather than acceleration along flow paths.…”

Historical and recent observations in polymer floods: An update review

“…Finally, Haynes, A. K., Clough, M. D., Fletcher, A. J. P., & Weston, S., (2013) reported that the rate at which fracture growth occurred was unknown. Choudhuri et al, 2013;Choudhuri et al, 2014;Koning and Mentzer, 1988;Shuaili et al, 2012;Thakuria et al, 2013;Zwaan and Valdez, 2015). This field was discovered in 1956 and started commercial production in 1980.…”

“…Generally, pressure increase observed in polymer injectors is associated to mobility reduction (oil bank formation), shear-thickening behavior of the injected polymer solutions (polymer rheology) and possible formation plugging effects due to poor water quality and/or polymer solubility, among others (Glasbergen G., Wever, D., Keijzer, E., & Farajzadeh, R., 2015; Laoroongroj, A., Zechner, M., Clemens, T., & Gringarten, A., 2012;Lin, Qiu & Guo, 2015;Lotfollahi et al, 2015;Shuaili et al, 2012;Van den Hoek et al, 2012). However, the potential impact of the formation of oil:water emulsion in the presence of polymer (i.e.…”

Operational Challenges and Monitoring of a Polymer Pilot, Matzen Field, Austria

“…A direct impact on the viscosity of the polymer solution could not be measured. However, because polymer injection occurred under fracturing conditions (Clemens et al 2013), the induced fractures are growing faster for poorer water quality (Suri et al 2011;Zechner et al 2015) as a result of more-extensive plugging of the fracture interfaces by the sludge. Because growing fractures might impact sweep efficiency if they are extensive (van den Hoek 2004), the water quality had to be improved to reduce this risk.…”