Saphir Guided Session #8

Transcription

1 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-1/11 Saphir Guided Session #8 A01 Introduction This Guided Session illustrates the minifrac option available for analysis of pressure data falloff. The minifrac analysis workflow will be explained and analyzed throughout one illustrative example. The new specialized plots will be used to perform first a post shut-in analysis and then an after closure analysis in order to obtain the closure parameters, to identify the various flow regimes and to assess the reservoir pressure and the far field transmissibility. B01 Opening the Saphir analysis The example under consideration here consists of a first period of water injection (with a rate of 2900 STB/D) lasting for hour followed by a 158 hours falloff. The injected zone is a gas bearing layer and has a pay thickness of 50 ft and the injected fluid is considered to be pure water. Note that the same procedure could be used if the formation was an oil bearing layer or an aquifer. In this last case the analysis would be of course monophasic. Start by opening the SapGS08.ks3 document. The file was created as follows: Gas was selected as the reference phase (it would have been Oil in case of oil formation or Water in case of aquifer) and water was declared as additional available rate. The Pay Zone set to 50 feet as shown in the screenshot below. All other parameters (including PVT parameters) were kept to their default values. Fig. B01.1 New document dialog

2 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-2/11 The PVT default values were used. The reservoir pressure was set to psia and the maximum pressure for the pseudo pressure calculation set at psia. The initial water saturation set to 12%. Fig. B01.2 New document PVT parameters dialog The rates were loaded using the Load Q icon in the control panel, selecting Keyboard the gas rate (or oil rate in case of oil bearing layer) was set to zero and the two steps for injection and falloff entered as follows. If the minifrac is performed on an aquifer zone, just input the monophasic water rate. Fig. B01.3 Well production definition The bottom-hole pressure gauge was loaded using Load P and the settings shown below:

3 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-3/11 Fig. B01.4 Pressure import dialog When opening the file, the pressure and water injection rate gauges are displayed in the browser. Groups have been created automatically. Click on the Edit Rates tab to display these gauges as well as the identified groups (injection followed by falloff), use the horizontal zoom button to zoom around the injection period: Fig. B01.5 Early time bottom-hole pressure and injection rate gauges We can now proceed with the analysis by extracting the falloff data. Extract the only available falloff keeping default extraction parameters.

4 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-4/11 Fig. B01.6 Extracted falloff B02 Minifrac analysis The minifrac analysis is available using the Minifrac button in the More tools control panel as soon as a falloff is present in the extracted data. If several groups have been extracted, only the falloff groups will be considered in the minifrac analysis. Click on the Minifrac button, this generates three new plots and adds the drawdown derivative to the existing Loglog plot. The new plots are called G-Function, Square root and ACA plots, we will use the first two to perform the post shut-in analysis and the last one for the after closure analysis. Fig. B02.1 Minifrac analysis

5 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-5/11 B02.1 Post Shut-in analysis The G-Function plot displays pressure P, derivative dp/dg and semi-log derivative G.dP/dG versus G-Function and is aimed at finding the fracture closure point. Maximize the plot, a new toolbar is now available,. The last five buttons activate the following minifrac functionalities: Fit a straight line that passes by the origin to the semi-log derivative G.dP/dG. Shift the dashed vertical line which represents the assessed closure point. Change the value of the fracture growth exponent. Change the smoothing parameter. Far field permeability estimate calculator. Note that if no minifrac analysis data exist for the current analysis, the vertical line which represents the closure point is set to an arbitrary value. This has to be changed by finding the point where the semi-log derivative departs from a straight line passing by the origin. In the present case, the pressure data shows an initial hump which is characteristic of a pressure dependent leak-off (PDL). Click on, select Draw through origin and click on a point of the semi-log derivative G.dP/dG where the trend is linear (in this case one could select a point corresponding to a G-value between 6 and 9, or equivalent to a time between 1.7 to 3.5 hr.). Click next on straight line: and drag the vertical dashed line to the point where data separate from the Fig. B02.2 G-Function plot The closure point has now been set based on the position of this vertical line. In order to check closure consistency, we will now use the Square root plot.

6 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-6/11 Minimize the G-Function plot and maximize the Square root plot. This plot is similar to the previous one but displays functions versus the square root of time. In this plot the closure point should correspond to a local maximum of pressure derivative. Use the vertical zoom button to zoom on the pressure derivative curve (in red here) around the chosen closure point and use the button to fine tune the closure to the nearest local maximum point (initial time data should be ignored here): Fig. B02.3 Square root plot Go back to the G-Function plot, maximize it and click in the toolbar on the button Show results in plot, the results should be: Pump time = hr. Pump volume = STB. Closure time = 3.07 hr. Closure pressure = psia. Closure G-value = G-Function (straight line) slope = psia. Additionally, secondary results are given such as Fracture efficiency (0.81) and Instantaneous Shut-In Pressure (ISIP) (12123 psia). We now have defined the closure point with the combined usage of the G-Function and Square root plots and have obtained all corresponding primary closure parameters. Note that in this example, the leak-off seems to be of PDL type, additional information such as fissure opening time and pressure can as well be inferred directly from the G-Function plot (in this case the observed fissure opening time is read directly as 1.4 hrs, corresponding to 5.3 G units). The next step is to focus on the after closure analysis.

7 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-7/11 B02.2 After Closure Analysis (A.C.A.) The after closure analysis is made with the last minifrac specialized plot called the ACA plot. In this plot the pressure difference (Pi-P) and semi-log derivative FL^2.d(P-Pi)/d(FL^2) are displayed versus the specialized square linear flow function (FL^2). In this plot, time is increasing from right to left. This function is highly dependent on the closure parameters and hence should be used after the closure has been properly assessed with the post shut-in analysis. Note also that the shape and amplitude of the pressure difference is very dependent on the reservoir pressure, while it does not affect the semi-log derivative curve. By default, the reservoir pressure is initialized to an arbitrary value (0.95 of the minimum pressure encountered during the reference fall-off). The plot is used for detecting possible characteristic flow regimes after the fracture closure. Two flow regimes are possible: pseudo-linear flow and/or pseudo radial flow: the former is observed when both pressure difference and the semilog derivative show characteristic half unit slope behavior and are separated by a factor 2 in the ACA plot, and the latter flow regime is recognized when both the pressure difference curve and the semilog derivative are joined in a single unit slope trend as time increases (data moving to the left on the x axis). Maximize the ACA plot, a new toolbar is available. Among the options there are seven minifrac options: Change the plot type from square linear flow function (FL^2) to linear (FL) or radial flow (FR) function. This option is used once the flow regimes have been identified (see below). Perform straight line slope regression to the pressure data (available only in linear/radial flow plots). Fit a graphical flow regime trend to the data (available only in square linear flow type, not used for parameters calculations, only visual aid for flow periods determination). Shift one of the dashed vertical lines which represent start and end of pseudo-linear and of pseudo-radial flow regimes. Hide/Show one of the vertical lines. Manually set the reservoir pressure. Change the smoothing parameter. We first start by identifying the flow regimes. Depending on the case, the default reservoir pressure value can be quite off (as it is the case in this example) which means that the pressure difference curve should not be considered initially and will be used later on to confirm the obtained reservoir pressure. In this example we have:

8 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-8/11 Fig. B02.4 Initial ACA plot From the shape of the semi-log derivative, we can identify a late pseudo-radial flow (unit slope). No clear pseudo-linear flow (1/2-slope) is present here. Hit the specialized line button, select Radial FR and set a graphical line onto the late (left part) semi-log data for putting into evidence the radial flow regime. We will remove the vertical lines corresponding to the start and end of the pseudo linear flow regime, as it cannot be detected here. Click on and uncheck start and end of linear flow, only one vertical line remains corresponding to the start to move this line toward the start of the pseudo- of the radial flow regime. Click next on radial trend: Fig. B02.5 Pseudo-radial flow regime identification Now that the flow regimes have been identified, click on plot type and select Radial FR (we will not use Linear FL as no clear linear flow regime has been detected). The plot has changed to the plot of pressure versus radial flow function in Cartesian coordinates. On this plot a straight line fit of the data in the radial flow regime time interval will give the reservoir pressure (and far-field mobility in the case of pseudo-radial flow).

9 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-9/11 Use the zoom on the horizontal axis button to focus on data around the picked start of pseudo-radial flow (recall here that time increases from right to left). Click on the line button, select Regression and set the two points for the linear regression (one on the left of the first data point and the second at the position of the dashed vertical line). The regression line appears and the reservoir pressure (10959 psia) is automatically updated based on the intercept of this line with the pressure axis. Fig. B02.6 Pseudo-radial flow line fit We can now go back to the square linear flow plot by clicking on and selecting Square linear FL^2. Note that the pressure and its derivative are now joining in a single unit slope trend, as the reservoir pressure Pi is now based on the radial flow straight line regression and is not an arbitrary value anymore. Fig. B02.7 Final ACA plot

10 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-10/11 In this case we have identified a pseudo-radial flow regime and calculated reservoir pressure and transmissibility. The main results are: Start of radial flow = 98 hr. Reservoir initial pressure = psia. Radial flow slope = psia. Far field mobility = md/cp. We now have obtained all possible primary parameters from the minifrac analysis; these are summarized in the analysis results dialog: Fig. B02.8 Minifrac analysis results B02.3 Use of the Loglog plot to confirm flow regimes Additionally, the typical loglog plot, used with the drawdown derivative, can be used to display and QC the observed flow regimes before and after the minifrac job. This is done by recognizing the characteristic slopes within the drawdown derivative. The drawdown derivative is preferred to the usual superposition derivative in minifrac analysis, and have different characteristic slopes for the minifrac flow regimes compared to the typical ones used for standard loglog plot analysis. - A slope of (+1/2) corresponds to a linear flow prior to closure. - A slope of (- 1/2) corresponds to linear flow after closure. - A slope of (-1) corresponds to radial flow after closure. From the minifrac analysis, we have obtained a fissure opening at 1.4 hours and a closure point at around 3 hours. The after closure pseudo-radial flow was observed after 100 hours and the inferred far-field mobility obtained was k/mu =0.084 md/cp.

11 Ecrin v Doc v KAPPA Saphir Guided Session #8 SapGS08-11/11 For minifrac analysis, because we are interested only in flow behavior occurring after shut-in time, we will set p@dt = ISIP for removing any unnecessary noises in the data due to frictional losses gradually stopping immediately after surface flow stops, and unrelated to the closure analysis. Right click in the loglog plot and choose change P@dt= 0 : The flow regimes can be observed on the Loglog plot by fitting specialized lines on the drawdown derivative. The dotted line indicates the time of closure, and we can fit a negative unit slope at late time corresponding to the observed pseudo-radial flow. We can also recognize in this plot a linear flow pattern prior to the fracture closure: Fig. B02.9 Loglog plot with drawdown derivative and a negative unit slope specialized line ( Minifrac Radial ). The obtained results can be seen in the results dialog of the Loglog plot and give from the late time radial flow a value of kh = 2.51 md.ft - note that since we are on a Loglog scale this value is very sensitive to the exact location of the specialized line. This corresponds to a far-field mobility of 0.05 md/cp in good agreement with the results obtained from the A.C.A plot used in minifrac analysis.