Best Practices - Available Field Methods

Injectivity and Falloff:

The well is closed in and buildup techniques developed for producers are applied. Basic analyses assume that the reservoir is homogeneous, constant thickness and contains a single fluid with small and constant compressibility. Before shut-in, water is injected at a constant rate through a well completely penetrating the target formation and the pressure is assumed to constant at a radius, re. This is the injectivity portion of the test. After constant rate injection, the well is shut-in and the pressure "falls off." Falloff analysis on this portion of the test can proceed with standard methods if certain assumptions can be made. Interpretation difficulties arise because of violations of these assumptions.
 

Average Reservoir Pressure:

Standard methods are available for inferring the average reservoir pressure. Modifications are required for closed systems. Simulation may be a necessity to account for offset production or injection.
 

Hall Plots:

A Hall plot is an injection well analysis technique that assumes instantaneous steady-state conditions. Volume and pressure data are required. Earlougher has modified the basic technique. If it is assumed that dimensionless pressure is independent of time, a relationship for a single well is:

where:

pwf ............... bottomhole pressure (psi)
t ............... time (hours, days, months)
pe ............... external pressure (psi)
µ ............... viscosity (cP)
pD ............... dimensionless pressure

 

Pulse Testing:

This is a form of interference testing (refer to Johnson, C.R., Geenkorn, R.A., and Woods, E.G.: "A New Method for Describing Reservoir Flow Properties Between Wells," JPT (December 1966) 1599-1604). Matthews and Russell stated "In this method a production well near the observation well is alternatively produced and then closed in to give a series of pressure pulses. The pulses are detected at the observation well by a very accurate gauge." Earlougher, 1977, is a good reference.

 

Interference Testing:

This is a multiple well transient test. It differs from a pulse test in that the duration of the stages is longer. A long duration rate modification in one well creates a pressure interference in an observation well that can be analysed for reservoir properties.

 

Step Rate Testing (SRT):

A step rate injectivity test is normally used to estimate the transition from matrix (or pseudo-matrix - a fracture may already be present but the bulk of the flow is radial) flow to fracture-dominated injection, according to a change in slope of a plot of pressure versus rate. Step rate testing allows for determination of when a new hydraulic fracture occurs and/or when a pre-existing fracture opens/propagates.

Step rate testing allows for determining when a fracture will propagate and when a pre-existing fracture will reopen. It can be run after a conventional falloff or a final falloff segment can be used in the test. Repeated falloff testing can also be used to assess if a reservoir has been altered by thermal changes in in-situ stresses or changes in kh associated with thermal effects.

Injection is carried out at a number of rates below fracturing pressure. At each rate injection continues until stabilization appears to occur. The injection operations are continued after indications of fracture opening/propagation. The opening pressure is inferred from a significant change in slope of a plot of bottomhole pressure versus injection rate.

 

Drillstem Testing (DST):

This is a temporary well completion designed to sample formation fluids, and usually also used to establish commercial viability of a producer. Drillstem testing (DST) can also be used in injection wells. Packers seal the appropriate interval and valves are operated to allow formation fluid to be collected. A pressure buildup can be obtained by closing the valves.

 

Hydraulic Impedance Testing (HIT):

Hydraulic Impedance Testing (HIT) is used to:
  1. detect hydraulic (or natural) fractures; intersecting a wellbore,
  2. determine the fracture closure pressure (indicative of the total minimum in-situ stress); and,
  3. to estimate fracture dimensions.

The test is performed by sending a pressure pulse down the wellbore (from the wellhead), and then analyzing the frequency response of the consequent wave train. HIT has been successfully used to determine fractures in numerous injection wells and the method is reported to work in both poorly consolidated and more consolidated formations.

 

Production Logging Testing (PLT):

One of the most useful diagnostics for determining injection coverage, the degree of fracturing, the degree of plugging is PLT testing. It becomes even more effective when it is combined with new-generation fiber optics sensing technologies.

 

Tracer Surveys:

This long-standing technology involves doping the injected fluid with a tracer and monitoring the time to breakthrough, the zone and the well(s) involved in breakthrough - useful for conformance control considerations.

 

Fibre Optics:

The British/European spelling is deferred to - to acknowledge the development efforts there. This is truly a technology that will (and is already starting to) revolutionize oilfield monitoring. Glass strands transmit signals and serve the dual purpose of being sensitive/continuous transducers themselves. Every individual involved in monitoring and control projects should be familiar with this technology.

 

Leakoff Testing:

A Formation Integrity Test is carried out to evaluate the integrity of a casing shoe. Pressure in the wellbore is increased to a specified level. If the hydraulic seal around the shoe does not fail, the only information that can be determined is a minimum level of the breakdown pressure.

Leakoff testing is carried out after drilling ahead a short distance from the casing shoe and pressurizing the openhole section. Usually, injection is stopped when there is an accelerated loss of fluid to the formation. Instrumentation and recording is generally minimal and the information is of limited precise value for inferring in-situ stress conditions. It is usually of more value for estimating tolerable equivalent circulating densities for drilling and completion. Extended leakoff testing, described below, is a formalization/extension of conventional leakoff testing where some legitimate in-situ stress information can be determined.

 

Extended Leakoff Testing:

This is strictly an extended version of conventional leakoff testing. After drilling ahead from the shoe a short distance, the openhole interval is pressurized until the formation breaks down. A fracture is created. Pumping continues to allow the fracture to propagate a short distance and the well is shut-in. This injection cycle is usually repeated once or twice more to establish if the shut-in pressure is relatively stable.

The pressure versus time behavior after shut-in is processed and the minimum in-situ principal stress is inferred from a significant inflection in the processed curve. Knowing the minimum in-situ principal stress, the formation pressure, and the breakdown pressure from the second or third injection, the maximum principal stress orthogonal to the wellbore can be determined.

For reasonable stress interpretation, rudimentary recording of (at least) surface pressure and rate versus time is necessary.

 

Microhydraulic Fracturing:

This is a technique that is fundamentally similar to Extended Leakoff Testing. It differs in that it can be done in open or cased hole (through perforations) at any time during the life of the well. Isolation is a pre-requisite. Depending on the open interval and its permeability, rates may not exceed 1/4 bpm.

 

Tiltmeter Measurements:

When a fracture is created at depth, the surrounding rock is deformed. There is a surface "reflection" of this deformation at depth as the surface tilts very slightly. Precision devices have been deployed around wellbores that are fractured and the tilt of the earth has been measured at strategic locations. Inverting these deformations provides a solution (not necessarily unique - judgment and fracture modelling may still be required) of the orientation, and dimensions of the fracture that was created.

 

Microseismic Monitoring:

When a discontinuity such as a fracture moves (surfaces moving relative to each other), when new fractures are created, etc., energy is released. The familiar analogy is an earthquake. Just as you can locate the focal point of an earthquake, you can measure much smaller energy emissions, logically called microseisms. These microseisms can be mapped to show chronologic changes in a reservoir that is being injected into.

 

Real-Time Diagnostics:

Starting in about 1979, several landmark papers by Ken Nolte and Mike Smith changed the way that we interpreted pressure time records for stimulation hydraulic fracturing treatments. The excess pressure during the pumping is plotted versus the time on a log-log scale. The slopes of this plot are taken as diagnostics of whether the fracture is contained in the target zone, whether there is substantial leakoff and whether or not screenout is occurring (and where). The same concepts can be adopted and modified for evaluating fractures that are grown during injection operations.

Nolte and Smith also initiated formalized analysis of the pressure-time curves after a treatment is shut-in and developed the original procedures for minifrac testing. These analyses allowed determination of the closure stress, the fluid loss coefficient (permeability and the degree of plugging can be inferred for injection hydraulic fractures) and possibly Young's Modulus. Guidelines were also provided for inferring whether natural fracture systems were opened up during the treatment.

 

Water Quality:

Various analytical measurements are available for assessing the "quality" of water to be injected. These include evaluations of the solids and oil present and the scaling potential.

 

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