Mini-Frac Treatment: Pre & Post Closure Data Analysis
This blog serves as the fourth installment in our Blog Series: Mini-Frac Treatment, The Key to Successful Hydraulic Fracturing.
In the last blog, we outlined the different terminologies used while conducting minifrac interpretation and discussed in details the use of step-down rate test to evaluate perforation & near-wellbore friction. In this blog, we will discuss the two different parts of pressure fall of data that is analysed post minifrac pumping. This categorization is made through the closure point, the point when fracture closes. Data recorded before this point and after pump shut-off is called pre-closure period, while data after closure point is called post-closure period.
Fall of period is mainly divided into two main parts:
Pre Closure Period
Post Closure Period
Pre Closure period is utilized to estimate Leakoff coefficient and fluid efficiency etc. Leakoff coefficient and fluid efficiency are the functions of rock matrix and fluid properties. While nature of rock matrix cannot be altered, fluid properties are such formulated that its leakoff properties support efficient frac creation and proppant placement. Too high leakoff properties would refer that fluid is leaking into formation faster than it technically should, which would consequently lower down net pressure and thus affecting net pressure and frac geometry. A simple way to calculate fluid efficiency as proposed by Nolte in 1980s is as following:
Where,
Tc = Closure Time, Tp = Pumping (Injection) Time
Fluid efficiency also determines the pad volume to be pumped prior to starting proppant stages. The dividing event between these two periods is obviously the formation closure. At this stage the net pressure inside fracture approaches zero and fracture closes under effect of its indigenous stress. Most widely used technique to identify the closure event is G-Function analyses.
The G-Function is a representation of the elapsed time after shut-in normalized to the duration of fracture extension. G-Function plot uses Cartesian coordinates and contains three curves versus the G-Function: pressure, derivative dp/dG, and semilog derivative G.dp/dG. The G-Function plot is first used to qualitatively characterize the leakoff behavior. The quantitative analysis then amounts to defining a line going through the origin and marking the departure from that line as the closure time. The plot results are the closure time, the closure pressure, the closure G function value Gc, the instantaneous shut-in pressure (ISIP) defined from a backward extrapolation of the pressure along the tangent at the closure point, and the Fluid Efficiency.
Typical G-function plot from fall-off data after mini-frac injection
Once the fracture closes, the pressure transient gradually starts shifting from pseudo linear to pseudo radial flow.
Nolte devised the technique to extract useful information including reservoir pressure, far field (matrix) permeability etc and named it as ACA (after closure analyses). The ACA plot includes fall-off pressure and its Bourdet derivative versus a squared function (dealing with pumping and closure time as estimated through G-Function). The analysis on the ACA plot requires the identification of developed pseudo-linear and/or pseudo-radial flow regimes, revealed by their characteristic slopes (½ for pseudo linear, 1 for pseudo radial). In the pseudo linear flow, dp and the semilog derivative should be parallel and their ratio should be 2; in the pseudo-radial flow regime both slopes should be 1 and the curves should coincide.
Typical After Closure Analysis plot on square linear flow x-axis
The After Closure Analysis can be also completed by an analysis using the conventional diagnostic plots: loglog, semilog, Horner.
Now that we understand the purpose of mini-frac, and overall mechanics behind the interpretation of its data, we stand a better ground to move forward in discussing a real case study, in which mini frac data is analyzed to estimate subsurface parameters.
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