Degree

Doctor of Philosophy (PhD)

Department

Craft and Hawkins Department of Petroleum Engineering

Document Type

Dissertation

Abstract

Naturally fractured reservoirs (NFRs) with bottom-water are known for their instant water breakthrough and severe water coning that reduces oil recovery. This is because water channels through the highly permeable fractures easily connecting the well to the aquifer bypassing the oil contained in the matrix. Remedial techniques such as producing below critical-oil rate, optimizing the well spacing and installing the downhole water sink (DWS)/ downhole water loop (DWL) technology, have already been successfully tested in single-porosity reservoirs (SPR). However, applicability of these techniques in NFRs are unknown since only a few studies have been performed on their feasibility in NFRs, to date. Moreover, metrics used for assessing severity of coning and performance of remedial techniques in single porosity reservoirs are insufficient or irrelevant for NFRs.

Two objectives of the study are: 1) to develop the water coning control design metrics specific for NFRs; and, 2) to optimize these metrics for oil recovery improvement in NFRs.

Critical oil rate is an important metric for coning severity. Consequently, the study develops a new semi-analytical “grey-box” model of critical oil rate for NFRs using the mechanistic model of single porosity reservoir and statistically calibrating it with the results from simulated experiments covering wide ranges of NFR. The study also relates critical rate to the placement of well’s completion in fracture network – in fractures or in rock matrix.

Based on the literature, natural fracture systems are classified as planar and channel (fracture corridor-type) fracture networks. A dual porosity/dual-permeability (DPDP) two-dimensional radial-cylindrical model is used for simulating planar networks and a 3-D Cartesian model - for simulating fracture corridor-type network. The study classifies the planar fractures as densely or sparsely distributed networks based on the minimum fracture spacing - critical fracture spacing – when the well’s placement (on or off fractures) has significant effect on the recovery.

The analysis of water-cut patterns in NFR identifies a stabilization stage that is a characteristic metric of the water coning process and can be controlled by well spacing design. The study correlates the duration of the stabilized WC stage with production rate and well-spacing, thereby providing a basis for optimizing NFR oil recovery.

Another metric – uniquely specific for NFRs – is the location placement of well’s completion within fracture network which controls the pattern and severity of water coning. The study compares recovery performance of well placement on/off-fractures for single and dual-completed wells. Due to uncertainty of well completion’s location with respect to the distributed fracture network, a field case of NFR is a studied to make probabilistic prediction of well’s oil recovery.

Well completion design – is an operational metric of water coning control. The study addresses the DWL well’s feasibility for on/off fracture well placement in NFRs.

Committee Chair

Wojtanowicz, Andrew K.

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