Residual gas saturation testing

In formations where the pore space is occupied by a stationary gas phase and a mobile water phase, such as in a watered-out gas reservoir, the residual gas saturation (S_gr) may need to be measured in situ. The S_gr also can be determined using a single-well injection/production test method.

Testing gas saturation

S_gr measurement involves injecting and immediately producing a suitable volume of water. The water used for injection typically is produced from the target well before the test and stored in tanks on the surface. During production, the amount of gas dissolved in the water (R_sw) that is produced from the formation is measured.

The injected water is essentially free of dissolved hydrocarbon gases (i.e., it is "dead"). As the injected water enters the formation that contains residual gas, the water dissolves the gas, becoming saturated at the temperature and pressure in the reservoir. A region of increasing radius around the wellbore is stripped of gas. Injection continues until a volume (V_I) has entered the formation. A material-balance tracer such as methanol is added to all injected water.

At the end of the injection, the stripped region near the well is a volume of pore space that is filled with dead water (V_d). Just beyond this dead-water zone, the pore space contains the original gas saturation and gas-saturated injected water.

Immediately after injection, water is produced back from the formation through the well to the surface. The first volume of water produced is the V_d from the leached region, and so contains no dissolved gas. This dead water is followed by gas-saturated water from the region outside the stripped zone. The gas-saturated water is able to move through the stripped region without giving up its dissolved gas to reform the original gas saturation. This nonreversing behavior is the reason the method works in a single well.

Calculating residual gas saturation

After the volume of dead water is produced, the observed gas content of the produced water rises to the solubility R_sw (scf/bbl) of gas at reservoir conditions. The volume of gas-saturated water (V_I – V_d) produced after the dead water contains the gas that was dissolved from the stripped zone. Thus, if B_g is the gas formation volume factor at reservoir conditions, the reservoir volume of gas dissolved in this fluid (V_g) is:

....................(1)

But this is just the volume of gas originally in the stripped zone, which is:

....................(2)

Equating the two and solving for S_gr:

....................(3)

Fig. 1 is a typical profile for gas content vs. produced water volume from a single-well injection test for S_gr. The volume required to produce the inflection point of the gas content profile is V_d. The final level of gas content is R_sw, the solubility of gas at reservoir conditions. These two values, along with the total injected volume V_I, are the only field data required to calculate S_gr. The procedure for measuring S_gr is described in detail by Bragg and Shallenberger.

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  Fig. 1 – Gas-saturation test example.

Applications

Performing the S_gr test is relatively simple. The only operational requirements are that a method must be available to produce water from the formation in the test well and that there must be a way to measure the gas content of the water leaving the formation, with which the fluid lifting mechanism does not interfere. Because of the nonreversing feature, the test cannot be repeated in a given completion. The gas that is dissolved from the near-well pore space is not replaced during the production step.

This method can be used to measure either hydrocarbon gas or carbon dioxide (CO₂) saturation after displacement by water. In some cases where residual oil, residual gas, and mobile water all share the pore space (three-phase system), this test has been conducted in tandem with the single well chemical tracer (SWCT) test. The ester solution and push volume are added to the end of the dead-water injection. A shut-in period also is added. The S_or value measured by the SWCT test is for the two-phase (oil and dead water), gas-leached zone near the well. The S_gr obtained from the gas content profile represents the fraction of the near-well pore space filled with gas before the test injection.

Nomenclature

Bg	=	formation volume factor, gas, RB/Mscf
Rsw	=	gas solubility in produced water at reservoir conditions, scf/bbl
Sgr	=	residual gas saturation, fraction of PV
Vd	=	volume of dead (gas-free) water produced, bbl
Vg	=	the reservoir volume of gas dissolved in gas-saturated water, scf/bbl
VI	=	volume of water injected, bbl

Noteworthy papers in OnePetro

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See also

Single well chemical tracer test

Residual oil evaluation using single well chemical tracer test

Connate water saturation evaluation

Well to well tracer tests

PEH:The_Single-Well_Chemical_Tracer_Test_-_A_Method_For_Measuring_Reservoir_Fluid_Saturations_In_Situ