Low-cost Monitoring of Inter-well Reservoir Communication Paths Through Correlations in Well Rate Fluctuations: Case Studies from Mature Fields in the North Sea

Abstract

A major key to rejuvenating mature oil and gas fields for improved recovery is the ability to characterize the reservoir between existing wells: currently practised methods for surveying inter-well properties are generally costly. A recent project has combined three technologies which utilize readily available fluid production histories to produce a process for identifying inter-well reservoir communications in producing oil or gas fields. Fluctuations in the well production and injection rate histories are analyzed in the context of coupled geomechanical-flow processes involving activated faults and fractures through (i) the Statistical Reservoir Model (SRM), (ii) conventional correlation analysis and a new technique for extracting rate diffusivity tensors, and (iii) coupled geomechechanical-flow modelling. These technologies have been applied to five oilfields located in the North Sea from the North Viking graben to the Central graben with the key results that (i) the general long-range nature of rate correlations is consistent with some dependence on hydro-mechanical changes near a critical point, and (ii) the patterns of orientational distributions of rate correlations from all five fields are consistent with induced shearing on faults or fractures as an inherent mechanism of reservoir communication. Identification of major reservoir pathways is of substantial advantage to efficiency in reservoir management, leading to benefit for practical issues of well placements and configurations, injectivities, productivities, sweep efficiencies, short-term and longer-term forecasting. In particular the techniques can be used in a time-lapse fashion in order to monitor changes in reservoir behaviour and provide real-time updating of reservoir models. Together, these techniques can directly assist rejuvenation of mature fields; additionally, albeit tentatively, the improved understanding of fundamental reservoir mechanisms can lead to better planning of ‘green’ fields.

The nature of stress state: numerical and laboratory experiments and some field observations

“…In the field, Heffer and co-authors (e.g. Heffer & Dowokpor 1990;Heffer et al 2010) have reported long-range injector-producer flow-rate correlations which cannot be explained by Darcy flow, but could be explained by a state of marginal stability (as defined by Jensen 1998) in which state the response to minor perturbations can be highly non-linear.…”

Geomechanical mechanisms involving faults and fractures for observed correlations between fluctuations in flowrates at wells in North Sea oilfields

“…1). This pattern suggests the involvement of some aspect of shear in the mechanism that provides signals between wells (Heffer et al 2010). It is pertinent that three of the six North Sea fields in the study are 'naturally fractured' in the conventional use of that term, while in the other three fields natural fractures have not previously been recognized as influential in the reservoir behaviour.…”

“…In these analyses the histories of flowrates at each well can be displaced in time relative to each other; for example, the flowrate at one well can depend not only on the current flowrates at other wells but also on flowrates at previous times, offset by a 'time-lag'. Main et al (2006Main et al ( , 2007 and Heffer et al (2010) described the application of the SRM and of standard correlation measures to the flowrate histories of six North Sea fields. These analyses found similar long-range fault-related stressrelated characteristics in the inter-well coefficients (all I-P, I -I and P -P wellpairs, where I is injection well and P is producer well).…”

“…These analyses found similar long-range fault-related stressrelated characteristics in the inter-well coefficients (all I-P, I -I and P -P wellpairs, where I is injection well and P is producer well). Heffer et al (2010) also introduced the concept of the flowrate diffusivity tensor and showed how components of these can be calculated for triangular elements between wells from the temporal and spatial decay of flowrate correlations calculated with time-lags of zero and one time-step. The flowrate diffusivity tensor is different from, but intuitively related to, the hydraulic diffusivity tensor which in turn is influenced by any fracturing.…”