Pressure monitoring during injection

There are methods for evaluation of pressure responses during acidizing. They are based on interpretation of recorded wellhead pressure values (or the subsequently derived values for bottom-hole pressure) and corresponding injection rates as treatment progresses. This is done to derive the evolution (hopefully, reduction) of the skin factor during matrix acidizing. Methods 

Service companies are equipped to provide on-site, real-time evaluation of acid treatments. However, because these techniques require accurate data, their usefulness is often limited. (These limitations have been discussed elsewhere. ) At the least, constant injection rates and an accurate pressuretime recorder are required. Despite its limitations, real-time evaluation is recommended in fields where multiple acid treatments are planned. It is most important to gather such data for initial treatments in a field. Future success may depend on it. Montgomery and colleagues have described techniques that can be used to apply real-time treatment evaluation to diagnostics and interpretation, enabling proper decision making and use of these methods.  

Analysis of flowback samples is important for observation of sludge, emulsion, solids production, and related problems. Compositional analysis of spent add (see chapter 16) can shed light on problems, espedally precipitation of add reaction products, and can be used to optimize future treatment design. 

Comparison of production rates (before and after acid treatment) is the most obvious and simple measure of success. Rate comparison should be made only after all spent acid has been returned and well production has retmned to formation fluids only. Changes in oil-to-water or gas-to-water ratios are important to monitor. Addizing should not preferentially stimulate a water zone. If that happens, it may be that the reservoir was not understood or that diversion was not effective. 

Poststimulation production data should be carefully compared to prestimulation data to verify that reported production rates are being maintained under comparable before-and-after conditions. There are many cases in which certain factors are not noticed or are misread, leading to a gross misinterpretation. For example, opening a surface choke (increasing drawdown), opening gas lift valves downhole, and changing a pump or surface 

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Numerous hydraulic tests were carried out, during the various project phases, either in an isolated borehole or between boreholes. Hydraulic experiments consisted of pulse tests, constant head injection and/or extraction and constant rate injection/extraction using borehole intervals delimited in FBX 95001, FBX 95002, BOUS 85.001 and BOUS 85.002 (Figure 1). Both water flow and piezometric level measurements for borehole intervals are available. Continuous pressure monitoring has allowed detecting crosshole responses during pulse testing these have then been used to calibrate the hydraulic conductivities of fractures. 

Donaldson and Johansen184 mention no monitoring wells at the site. About 6 years after injection began, pressure interference from the two injection wells was observed. During the same period, the fluid front from Well No. 1 was about 223 m (730 ft) from the well bore. 

During the 2007 field experiments, C02 concentrations of the water samples under the reservoir pressure could not be correctly monitored due to the broken of the water sampler. This means that water samples were collected around 800m depth and a part of C02 must be degassed during ascending to the surface from the sampler. Therefore, C02 concentrations of the reservoir fluids are calculated on the basis of the observed pH and charge balance of each samples (Case 1) and the tracer concentration (Case 2). The Case 2 means that C02 in the injected water did not react with rocks. Fig.3 shows the calculated C02 concentration with iodine. Iodine concentration decreases from 1000 to 200pg/L with the elapsed time. From Fig.4, the fraction of the injected C02 water (1 wt.% C02) is almost zero. This means that... 

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