Abstract

Produced water re-injection (PWRI) for use in pressure maintenance is an alternative to discharge to the environment and is increasingly practised industrially in view of the combination of ever more stringent discharge standards and high water treatment costs.

Due to the damaging effects on formations of produced waters it is often necessary to inject in fracturing regime to maintain the desired injection rate. Then the main issues of the process are the impact on well injectivities, fracture growth and sweep efficiency.

All these issues are addressed by presenting an onshore field case of PWRI on a low permeability carbonaceous reservoir. Aquifer water has been injected for ten years and progressively substituted by produced waters for the next ten years. This case is well monitored and documented (fall-off and step rate tests, temperature logs, water quality). The main parameters affecting the process are illustrated in diagnostic analysis and the injectivities are satisfactorily reproduced using different simulation techniques. Special care is taken to describe filter cakes properties and distributions behind the fracture faces and inside the de fracture in order to well define the effects on the loss of fracture injectivity. The resulting fracture dimensions are very consistent with the last fall-off measurements and analysis.

Finally simulating incrementally this fracture growth in the Eclipse reservoir model it was possible to demonstrate that water breakthrough occurs when the fracture tip reaches laterally higher permeability zones in the producer area and to correlate decreases of water production with injection shut in periods associated with fracture tip closure.

Introduction

The case presented is an onshore field at 3000 m depth and situated in the south west of France. The reservoir is a very hard carbonate with a Young modulus of 500000 bars. Very significant lateral heterogeneities are existing. The permeabilities in the injection area are less than 1 mD compare to 1–10 mD in the producing area with effective permeabilities of 50–2100 mD due to a natural fissure and fracture network. The basic data at the beginning of injection on 1979 for one typical well are given in table 1. Fresh aquifer waters from another field has been injected peripherial (figure1a) for pressure support at high pressures during 10 years followed by commingle produced water reinjection during another 10 years.

Due to the strange behavior of the well different monitoring techniques has been used over the life of the field leading to interpretation difficulties. With the help of the recent improved knowledge on fractured injection regimes and reinjection effects (Ref. 1 to 7) all the field data has been reprocessed and the results of these analysis are discussed in the following chapters.

Historical diagnostic analysis

The entire historical is presented in figure 1.The different injection periods and tests are mentioned. The first surprise is that at first look there is apparently no effect of produced water re-injection on injectivity. Transforming surface in bottom-hole pressure does not change this observation.

Reservoir pressure effects.

In fact the reservoir pressure is decreasing significantly during the produced water injection period as shown on figure 2. Then plotting the evolution of the differential pressure (bottom-hole pressure - reservoir pressure) with time on figure 3 shows a significant decrease in injectivity during this last period (increase in pressure for the same rate of 400 m3/d).

Pressure rate plot.

When the reservoir pressure is varying this plot must also be done using differential pressures otherwise this can lead to big mistakes in the diagnostic of injection regimes.

Figure 4 shows that the injection regime is fracturing excepted at the early beginning of injection where the points close to the Y axe correspond to radial flow.

Reservoir pressure effects.

In fact the reservoir pressure is decreasing significantly during the produced water injection period as shown on figure 2. Then plotting the evolution of the differential pressure (bottom-hole pressure - reservoir pressure) with time on figure 3 shows a significant decrease in injectivity during this last period (increase in pressure for the same rate of 400 m3/d).

Pressure rate plot.

When the reservoir pressure is varying this plot must also be done using differential pressures otherwise this can lead to big mistakes in the diagnostic of injection regimes.

Figure 4 shows that the injection regime is fracturing excepted at the early beginning of injection where the points close to the Y axe correspond to radial flow.

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