Page 258 - Air and gas Drilling Field Guide 3rd Edition

10.1 Stable Foam Rheology 249

of a proposed mixture of water, surfactant, and additives in a future drilling project,
it is necessary to (1) establish a nominal baseline laboratory assessment of a stable
foam mixture that has undergone extensive laboratory experimental assessments
and has a record of good performance in the field; (2) devise and use a simple lab-
oratory test that could be used to measure other potential stable foam mixtures
against the nominal baseline mixture; and (3) create an empirical relationship that
can be used to predict the future performance and, more importantly, the future
nonperformance of nonbaseline drilling project mixtures. In this stable foam per-
formance screening, the quality of the water used in a drilling project cannot be
overemphasized. Therefore, it is very important that the actual water used in a
forthcoming drilling operation be used in these laboratory screening tests.
A screening test used by a few of the companies is a highly modified version
of an ASTM foam properties test [17]. The test requires the use of a commercial
blender and a tall 1000-ml beaker. The actual mixture of drilling project water,
surfactants, and additives is to be used in the test. One hundred milliliters of
the drilling project water with its additives (excluding the surfactant) is placed
in the blender. The appropriate quantity of surfactant is placed in the blender
with the water plus additives mixture. The blender is operated with an open
top for 10 min. Stable foam will be created as the blender is operated. At the
end ofthe blending time,aspatulaisusedtopour all thestablefoaminthe
blender into the beaker. The total “height” in milliliters of the mixture is
recorded (e.g., 500 ml). A stop watch is used to obtain the time needed for
50 ml of liquidtoaccumulate at thebottomofthebeaker.Thistimeisknown
as the “half-lifetime” of the foam mixture and is recorded in seconds (e.g.,
200 sec). For illustration, these values will be considered “nominal baseline”
for a stable foam mixture that has shown good performance characteristics in
extensive laboratory dynamic flow tests and in field drilling project results.
Using Equation (10-1), the foam quality of the nominal baseline stable foam mix-
ture would be G ¼ 0.80.

10.1.2 Empirical Algorithm for Screening
Most screening empirical algorithms are constructed on the basis of the height
and half-life ratios of the proposed drilling project mixture values to the nominal
baseline mixture values. The expression given here is an example of such an
empirical algorithm. In this expression, L is the value used to multiple the effec-
tive viscosity of the nominal baseline stable foam mixture.
a
b
h t t t
L ¼ C test ; (10-4)
h n t n
where h t is the height of the test mixture (ml), h n is the height of the nominal
baseline mixture (ml), t t is the half-life time of the test mixture (sec), t n is the
half-life of the nominal baseline mixture (sec), and C test ,a, and b are constants
determined by dynamic laboratory experiments or field data.