Advanced Reservoir Engineering (eBook)

Tarek Ahmed, Paul McKinney (Autoren)

eBook Download: EPUB

2011 | 1. Auflage
424 Seiten
Elsevier Science (Verlag)
978-0-08-049883-6 (ISBN)

Infos

Advanced Reservoir Engineering offers the practicing engineer and engineering student a full description, with worked examples, of all of the kinds of reservoir engineering topics that the engineer will use in day-to-day activities. In an industry where there is often a lack of information, this timely volume gives a comprehensive account of the physics of reservoir engineering, a thorough knowledge of which is essential in the petroleum industry for the efficient recovery of hydrocarbons.

Chapter one deals exclusively with the theory and practice of transient flow analysis and offers a brief but thorough hands-on guide to gas and oil well testing. Chapter two documents water influx models and their practical applications in conducting comprehensive field studies, widely used throughout the industry. Later chapters include unconventional gas reservoirs and the classical adaptations of the material balance equation.

* An essential tool for the petroleum and reservoir engineer, offering information not available anywhere else


* Introduces the reader to cutting-edge new developments in Type-Curve Analysis, unconventional gas reservoirs, and gas hydrates

* Written by two of the industry's best-known and respected reservoir engineers

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1.3 Transient Well Testing

Detailed reservoir information is essential to the petroleum engineer in order to analyze the current behavior and future performance of the reservoir. Pressure transient testing is designed to provide the engineer with a quantitative analysis of the reservoir properties. A transient test is essentially conducted by creating a pressure disturbance in the reservoir and recording the pressure response at the wellbore, i.e., bottom-hole flowing pressure pwf, as a function of time. The pressure transient tests most commonly used in the petroleum industry include:

• pressure drawdown;

• pressure buildup;

• multirate;

• interference;

• pulse;

• drill stem (DST);

• falloff;

• injectivity;

• step rate.

It should be pointed out that when the flow rate is changed and the pressure response is recorded in the same well, the test is called a “single-well” test. Drawdown, buildup, injectivity, falloff, and step-rate tests are examples of a single-well test. When the flow rate is changed in one well and the pressure response is measured in another well(s), the test is called a “multiple-well” test.

Several of the above listed tests are briefly described in the following sections.

It has long been recognized that the pressure behavior of a reservoir following a rate change directly reflects the geometry and flow properties of the reservoir. Some of the information that can be obtained from a well test includes:

Drawdown tests

Pressure profile

Reservoir behavior

Permeability

Skin

Fracture length

Reservoir limit and shape

Buildup tests

Reservoir behavior

Permeability

Fracture length

Skin

Reservoir pressure

Boundaries

DST	Reservoir behavior

Permeability

Skin

Fracture length

Reservoir limit

Boundaries

Falloff tests

Mobility in various banks

Skin

Reservoir pressure

Fracture length

Location of front

Boundaries

Interference and pulse tests

Communication between wells

Reservoir-type behavior

Porosity

Interwell permeability

Vertical permeability

Layered reservoir tests

Horizontal permeability

Vertical permeability

Skin

Average layer pressure

Outer boundaries

Step-rate tests

Formation parting pressure

Permeability

Skin

There are several excellent technical and reference books that comprehensively and thoroughly address the subject of well testing and transient low analysis, in particular:

• C. S. Matthews and D. G. Russell, Pressure Buildup and Flow Test in Wells (1967);

• Energy Resources Conservation Board (ERCB), Theory and Practice of the Testing of Gas Wells (1975);

• Robert Earlougher, Advances in Well Test Analysis (1977);

• John Lee, Well Testing (1982);

• M. A. Sabet, Well Test Analysis (1991);

• Roland Horn, Modern Well Test Analysis (1995).

1.3.1 Drawdown test

A pressure drawdown test is simply a series of bottom-hole pressure measurements made during a period of flow at constant producing rate. Usually the well is shut in prior to the flow test for a period of time sufficient to allow the pressure to equalize throughout the formation, i.e., to reach static pressure. A schematic of the ideal flow rate and pressure history is shown in Figure 1.32.

Figure 1.32 Idealized drawdown test.

The fundamental objectives of drawdown testing are to obtain the average permeability, k, of the reservoir rock within the drainage area of the well, and to assess the degree of damage of stimulation induced in the vicinity of the wellbore through drilling and completion practices. Other objectives are to determine the pore volume and to detect reservoir inhomogeneities within the drainage area of the well.

When a well is flowing at a constant rate of Qo under the unsteady-state condition, the pressure behavior of the well will act as if it exists in an infinite-size reservoir. The pressure behavior during this period is described by Equation 1.2.134 as:

where:

k = permeability, md

t = time, hours

rw = wellbore radius, ft

s= skin factor

The above expression can be written as:

[1.3.1]

This relationship is essentially an equation of a straight line and can be expressed as:

where:

and the slope m is given by:

[1.3.2]

Equation 1.3.1 suggests that a plot of pwf versus time t on semilog graph paper would yield a straight line with a slope m in psi/cycle. This semilog straight-line portion of the drawdown data, as shown in Figure 1.33, can also be expressed in another convenient form by employing the definition of the slope:

Figure 1.33 Semilog plot of pressure drawdown data.

or:

Notice that Equation 1.3.2 can also be rearranged to determine the capacity kh of the drainage area of the well. If the thickness is known, then the average permeability is given by:

where:

k = average permeability, md

|m| = absolute value of slope, psi/cycle

Clearly, kh/μ or k/μ may also be estimated.

The skin effect can be obtained by rearranging Equation 1.3.1 as:

or, more conveniently, if selecting pwf = p1hr which is found on the extension of the straight line at t = 1 hr, then:

[1.3.3]

where |m| is the absolute value of the slope m.

In Equation 1.2.3, p1hr must be obtained from the semilog straight line. If the pressure data measured at 1 hour does not fall on that line, the line must be extrapolated to 1 hour and the extrapolated value of p1hr must be used in Equation 1.3.3. This procedure is necessary to avoid calculating an incorrect skin by using a wellbore-storage-influenced pressure. Figure 1.33 illustrates the extrapolation to p1hr.

Note that the additional pressure drop due to the skin was expressed previously by Equation 1.2.130 as:

This additional pressure drop can be equivalently written in terms of the semilog straight-line slope m by combining the above expression with that of Equation 1.3.3 to give:

Another physically meaningful characterization of the skin factor is the flow coefficient E as defined by the ratio of the well actual or observed productivity index Jactual and its ideal productivity index Jideal. The ideal productivity index Jideal is the value obtained with no alternation of permeability around the wellbore. Mathematically, the flow coefficient is given by:

where is the average pressure in the well drainage...

Produkt-Details

• Erscheint lt. Verlag: 15.3.2011
• Sprache: englisch
• Themenwelt: Sachbuch/Ratgeber
• Themenwelt: Technik ► Bauwesen
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 0-08-049883-3 / 0080498833
• ISBN-13: 978-0-08-049883-6 / 9780080498836