8. Treatment Evaluation

7/23/2019 8. Treatment Evaluation

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Treatment Evaluation

Post-job evaluation involves

routinely collecting and analyzing all information relating to the fracture treatment

periodically evaluating the wells post-frac performance in relation to

those of other wells in the field

comparing the predicted treatment results with the actual post-frac

performance

identifying problems that occurred and determining how they could have

been avoided

A thorough evaluation of each treatment enables us to incorporate improvements into

subsequent treatments in the same or similar fields, and prevent the recurrence of identical

problems.

Post-Treatment Fracture Height Determination

Temperature surveys, which are used to determine the fracture height at the

wellbore, are among the most useful tools employed in post-treatment

evaluation. A temperature survey is conducted shortly after pumping is

completed. It measures the change in bottomhole temperature that has taen

place because of the large volume of fluid injected into the formation. !ore and

more often, this method is being replaced or supplemented by radioactive tracer

logging. "oth techni#ues have been successfully used to determine fracture

height, although they are both subject to errors in interpretation, mostly because

they provide information only on the fracture height at the wellbore and not in the

formation.

Post-Treatment Determination of Fracture Conductivity and Length

In low permeability formations, it is often not possible to run a pressure transient

test before the fracturing treatment, because the well will not produce without

fracturing. In such a case, a pressure buildup test of the fractured well is used to

obtain the permeability and the fracture e$tent simultaneously. %nfortunately, this

is an ill-posed problem in the sense that many different combinations of the

unnown parameters give a good fit. In high permeability formations, the

permeability is usually nown and the primary goal of a post-treatment test is to

evaluate the created fracture.

The transient behavior of a vertical well intersected by a finite conductivityfracture for infinite- acting reservoir is well nown due to the wors of &inco-'ey


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et al. ()*+, )*).

Figure 1

igure )(Log-log plot of dimensionless pressure for a vertically fractured well.

After Cinco-Ley and Samaniego, 1978 shows the log-log plot of the

dimensionless pressure and the /time-log derivative/ parameteri0ed by the

dimensionless fracture conductivity. In the so called bilinear flow regime, where

the flow is determined by both the reservoir and fracture properties, the plot

shows a #uarter slope because in this flow regime the dimensionless pressure can

be e$pressed as

(1)

where tDxfis the dimensionless time based on the fracture half-length:
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(2)

nce we identify such a regime, we can construct a specialized plot of the pressure versus

the quarter root of time. !he slope, mbf, of the straight line fitted is a combination of the

reservoir and fracture properties:

(3)

"t is obvious from the above equation that we cannot simultaneously determine the formation

permeability and the fracture conductivity from this regime. #nowing the formation

permeability, we can determine the dimensioned fracture conductivity $kfw% from the slope, but

not the fracture extent. ur suggestion is to assume CfD& '.(, determine an equivalent

dimensioned fracture conductivity based on)quation*,

(4)

and calculate an equivalent fracture length according to:

(5)

+omparing the equivalent fracture length to the design length may provide valuable

information on the success of the fracturing ob.

The bilinear flow period ends before t1$f 2 3.3), which may be a very short timefor higher permeability reservoirs. If we cannot identify the bilinear flow regime

from a well test, the best approach is to determine the Productivity Inde$ from

pseudo-radial flow analysis, assuming either infinite-acting or pseudo-steady-

state behavior. The Productivity Inde$ can be obtained from a build-up test, a

multiple-rate test, or else from production data.

4e would hope that the post treatment Productivity Inde$ indicates an

improvement with respect to the theoretically undamaged (0ero-sin behavior. If

there is no such improvement, we cannot evaluate the treatment in terms of

having created a fracture. 5till, the treatment may prove economically successful


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simply by having reduced the sin to a smaller positive value. Assuming that the

Productivity Inde$ indicates a negative sin, and accepting proppant permeability

and formation permeability values, we can determine the theoretical volume of

proppant that we have to place into the formation to obtain the same Productivity

Inde$. &omparing the theoretically necessary volume of proppant with thevolume of proppant actually injected, we obtain an overall efficiency-type

indicator. In addition, we can brea down the theoretically necessary volume into

a length and width using the optimum dimensionless fracture conductivity. 4e

can then visuali0e the created effective fracture length, the same way as we did

above for the bilinear flow analysis.

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8. Treatment Evaluation