Step Rate Testing

Cardinal Surveys Company 2009

Steve McLaughlinPresidentCardinal Surveys Company



You do not want to frac your injectorsOld technology, but must be done rightScant literature, but agrees on fundamentalsMost costly waterflood mistake: Unintentional Fracturing and direct channelsMost overlooked, underutilized technologyRapid, premature breakthrough still plagues new floods todayState and Federal regulations dictate, provide some guidance


Goal: Reliable estimate of FPP (Formation Parting Pressure)

Injection above FPP: Premature breakthrough of injected fluidsPoor sweep efficiencyReduced recoveryLoss of costly injection fluids.

Injection far below FPP:Injection volumes much lower than the allowable maximum Reduced rate of oil recover.


“A reliable estimate of the formation parting or fracture extension pressure is important for efficient operation of water floods and tertiary recovery projects.”

• - Pramod Singh, SPE, and Ram G. Agarwal, SPE, Amoco Production Co. SPE 18141, 1990.


“Rapid water breakthrough in production wells as a direct result of exceeding a certain critical injection pressure in nearby injection wells was observed as early as 1945.(1) This critical injection pressure is called the formation or fracture parting pressure (FPP). The FPP is equivalent to the ‘fracture extension/propagation pressure’ in the hydraulic fracturing literature.(2) Several studies (3-5) recently demonstrated that a fracture will propagate if injection is above the FPP and the injection/withdrawal ratio is greater than one. In addition to this uncontrolled fracture extension, injection above the FPP may also cause fracturing out of pay. These factors may lead to premature breakthrough of injected fluids, poor sweep efficiency, reduced recovery, and loss of costly injection fluids. “

Cont.


“On the other hand, injection far below the FPP may result in injection volumes much lower than the allowable maximum and a reduced rate of oil recover. A reliable estimate of the FPP is therefore critical in conducting secondary and tertiary recovery projects.(6)”

• - Pramod Singh, SPE, and Ram G. Agarwal, SPE, Amoco Production Co. SPE 18141, 1990.


What is a “Step Rate Test (SRT)?”A series of constant-rate injections (“Steps”) increasing from low to high, designed to determine the Formation Parting Pressure (FPP).Formation Parting Pressure is the pressure which will initiate formation fracture.Fracture is undesirable when it leads to by-passing productive pay and channeling directly to adjacent producers.


FPP vs. closure pressure:Closure pressure is less than the Formation Parting Pressure.Closure pressure can be determined by in-situ stress testing methods.Rules of Thumb usually limit injection pressure to about 80% of FPP which should also be below closure pressure.


Eaton’s Method, 1969

rr

P)1(

)P(

e

ePFG

ob

FG = Fracture GradientPob = Overburden GradientPr = Reservoir Pressure GradientE = Poisson’s Ratio


Eaton’s Method – West Texas Example

Pob = 1 PSI/ft (overburden gradient)Pr = 0.433 PSI/ft (fresh water reservoir gradient)E = 0.25 (Poisson’s ratio)

FG = 0.622 PSI/ft

4,500’ well = 2,799 PSI FPP


Note: Eaton’s Method from example using 80% rule-of-thumb equals 0.5 psi/ft gradient:

0.622 X 0.80 = 0.5 psi/ft

This is the Texas Railroad Commission default permitted value.


Measuring pressure – Down Hole or Surface?Theoretically, pressure can be measured at the surface, and corrected for hydrostatic and frictional factors.Practically, only down-hole (near formation face) placed measurements yield good SRT data.Real-time monitoring required to insure quality of the SRT, and to insure FPP has been reached.


Real – Time Injection Rate Steps


Real – Time Bottom Hole Pressures


Injection Rates vs. BHP




New Mexico

OCD (Oil Conservation Division)

UNDERGROUND INJECTION CONTROLPROGRAM MANUALFebruary 26, 2004


New Mexico OCD rules:Injection Pressure:• The initial permitted injection pressure is limited to

0.2 psi/ft.• Wells may be permitted at a higher initial injection

pressure at the time of application if adequate step-rate test information is provided to the Division


New Mexico OCD rules:Pressure Increases (over 0.2 PSI/ft):• The Division may authorize an increase in injection

pressure upon a proper showing by the operator that such higher pressure will not fracture the formation and will not result in migration of injected fluids from the authorized injection zone. Such proper showing shall consist at least of a valid step-rate test run in a manner acceptable to the Division


Texas RRC Statewide Rule 46

To ammend Injection Pressure above 0.5 PSI/ft, a Step Rate Test is required.


Step Rate Test Guidelines – Texas RRCMust demonstrate that formation fracturing will not occur at the proposed injection pressure. SRT used to demonstrate.Measure both surface and BHP.The test well should be shut in long enough so that the bottom-hole pressure is near the shut-in formation pressure (No less than 48 hours.).


Step Rate Test Guidelines – Texas RRCThe well may need to be backflowed if the shut-in pressure is above the expected fracture pressure of 0.5 psi/ft. of depth. Suggested rates for the test are 5, 10, 20, 40, 60, 80, and 100% of the proposed maximum daily injection volume and corresponding pressures. The first rate should be the formation matrix rate (the rate at which the formation begins to accept fluid).


Step Rate Test Guidelines – Texas RRCAt least two rates must be below the expected fracture pressure of 0.5 psi/ft. of depth. Each rate must be allowed to stabilize before proceeding to the next higher rate. (60 min for formation permeability < 10 millidarcies, and 30 min for perm > 10 millidarcies.)


Step Rate Test Guidelines – Texas RRCEACH STEP SHOULD LAST EXACTLY AS LONG AS THE PRECEDING STEP.Once the fracture pressure is reached, continuing the test by stepping downward will refine the fracture pressure and provide quality control for the test.

http://www.rrc.state.tx.us/forms/publications/HTML/fsrt.php


United States Evironmental Projection Agency (EPA)Underground Injection Control regulations promulgated in response to requirements of the Safe Drinking Water Act include 40 CFR 146.13(a) for non-hazardous waste wells and 146.67(a) for hazardous waste wells, which state that: Except during stimulation, the owner or operator shall assure that injection pressure at the wellhead does not exceed a maximum which shall be calculated so as to assure that the pressure in the injection zone during injection does not initiate new fractures or propagate existing fractures in the injection zone.

http://www.epa.gov/R5water/uic/r5guid/r5_07.htm


United States Evironmental Projection Agency (EPA)Recommends Fracture Closure Pressure as maximum injection pressure.Prefers in-situ stress testing to SRT. (In-situ stress testing is where fluid is injected above the break-down pressure [fracture pressure], and then pressures are monitored after fluid injection is stopped to determine closure pressure and pore pressure. The pumping cycle is often repeated multiple times.)


EPA STEP-RATE TEST PROCEDURE:1) The well should be shut in long enough prior to testing such that the bottom hole pressures approximate shut-in formation pressures. If the shut-in well flows to the surface, the wellhead injection string should be equipped with a gauge and the static surface pressure read and recorded.2) A series of successively higher injection rates are determined using guidelines below, and the elapsed time and pressure values are read and recorded for each rate and time step. Each rate step should last exactly as long as the preceding rate. If stabilized pressure values are not obtained within the rate steps suggested below, the test results may be considered as inconclusive.

http://www.epa.gov/region8/water/uic/INFO-StepRateTest.pdf


EPA STEP-RATE TEST PROCEDURE:Formation Permeability (md) Total time per rate-step (min)

≤ 5 md 60 min≥ 10 md 30 min

3) Suggested injection rates (percent of maximum anticipated injection rate):

5% , 10%, 20%, 40%, 60%, 80%, 100%4) Injection rates should be controlled with a constant flow regulator that has been tested prior to use. A throttling device is not considered sufficient.5) Flow rates should be measured with a calibrated turbine flowmeter.



EPA STEP-RATE TEST PROCEDURE:

6) Record injection rates using a chart recorder or a strip chart.7) Measure pressures with a down hole pressure bomb. If a surface gauge is used, the test pressures must be corrected for the estimated friction loss at each particular flow rate.8) Measure and record injection pressures with a gauge or recorder (for immediate test results). Record each time step and corresponding pressure.




9) A plot of injection rates and the corresponding stabilized pressure values should be graphically represented as a constant slope straight line to a point at which the formation fracture, or “breakdown”, pressure is exceeded. The slope of this subsequent straight line should be less than that of the before-fracture straight line (see example).




10) If the formation fracture pressure has definitively been exceeded, as evidenced by at least two injection rate-pressure combinations greater than the breakdown pressure, the injection pump can be stopped, and the line valve closed and pressure allowed to bleed-off into the injection zone. There will occur a significant instantaneous pressure drop (Instantaneous Shut-in Pressure or ISIP), after which the pressure values will level out. This ISIP value must be read and recorded. The ISIP obtained in this manner may be considered to be the minimum pressure required to hold open a fracture in this formation at this well..




11) Once the ISIP is obtained, the SRT is concluded.12) In the event that the breakdown pressure was not obtained at the maximum test injection pressure utilized, the test results may indicate that the formation is accepting fluids without fracturing.



EPA SRT Example


Other method:2 Step MethodSPE 18,141 – Singh and Agarwal, 1989.U.S. Patent No. 4,793,413 – Singh, Agarwal and Miller, 1988.

• Uses continuous recording of rate and pressure data.• Step one below FPP, Step two above FPP.• Delta time vs Delta pressure plots determine FPP.


Examples Follow

Documents

Step Rate Testing