Field tests are regularly carried out for formation damage monitoring. Using appropriate precursors or signatures for detection of the imminent formation damage problems and their impact on oil production rates are important for two reasons, as stated by Hayatdavoudi (1999):

1. It would help us understand the reasons for a premature drop in oil production or formation damage.

2. It would give us adequate time to take the necessary, preventive remedial action(s) prior to the onset of a serious damage or an unusual drop in oil and gas production.

In this chapter, the loss of productivity or injectivity of wells by formation damage is expressed by alternative measures, and the various tests available for measurement and diagnosis of formation damage problems in the field are described.

Diagnosis and Evaluation of Formation Damage in the Field

As stated by Yeager et al. (1997), "No individual test or tool can provide the only information needed for damage mechanism identification and evaluation, and a historical perspective rather than an isolated perspective will result in a more complete diagnosis of the presence and type of damage." Therefore, Yeager et al. (1997) further elaborate that "Damage mechanism identification requires a systematic approach to research, planning, and evaluation of all available information." For the most part, the diagnosis and measurement of formation damage in the field rely on well testing, well logging, history matching, and produced fluid analysis. The determination of the mechanisms responsible for loss of flow efficiency (productivity or injectivity) requires a number of studies. Yeager et al. (1997) recommend a three-stage approach consisting of:

1. Quantifying the degree of existing damage

2. Diagnosing the existing downhole damage mechanisms

3. Performing laboratory studies to increase knowledge about specific mechanisms

For this purpose, formation damage studies begin with the classification of the reservoir formation and review of the operational and engineering processes. The flow chart given by Yeager et al. (1997) in Figure 22-1 describes the information required on various aspects. The methodology for determination of the mechanisms responsible for flow efficiency is described in the flow chart given in Figure 22-2 by Yeager et al. (1997). Although, the Yeager et al. (1997) approach is intended for identification of damage in gas-storage wells, its applicability is extended here for general applications. As stated by Yeager et al. (1997), the typical downhole diagnostic tests that can be conducted in the field include:

1. Well-test analysis to determine quantitatively if damage exists

2. Downhole video to observe the wellbore and formation areas

3. Physical sampling in the form of downhole liquids and solids

4. In the openhole completions, rotary sidewall core samples of the wellbore face as a "biopsy" of the storage formation

Yeager et al. (1997) recommend a downhole video run prior to the other downhole diagnostic tests and sampling for assessment of the presence, nature, and morphology of deposits on the wellbore surface and

perforations. Figure 22-3 by Yeager et al. (1997) shows a schematic of a typical high-resolution video camera and a still image, indicating significant wellbore scaling, obtained using this camera. The video observations also provide valuable information necessary for determination of the flow distribution that can be used to improve the accuracy of the welltest interpretation and identification of the formation damage mechanisms (Yeager et al., 1997).

Pressure transient tests yield information on the permeability and formation thickness product, (Kh), and skin factor, s. As pointed out by Yeager et al. (1997), pressure transient tests only provide information at a specific time, when the tests are conducted. Therefore, formation damage can be more effectively evaluated by conducting a series of tests over a length of time and also the true skin should be determined after corrections for other effects, such as non-Darcy or inertial effects (Yeager et al., 1997).

In openhole completed wells, core samples can be taken from the wells using a rotary sidewall coring tool (Yeager et al., 1997). The material on the face of the extracted cores should be carefully preserved during the transportation of the core for later analytical studies (Yeager et al., 1997).


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