analysis of fluids and solids mineralogy and fabric from flowback waters from
fifty Montney Formation horizontal well completions in Western Canada, when
coupled with petrophysical and lithological analysis of core, provides insights
into the reservoir geology, which in turn enables strategies for optimising
well completions, production, and well surveillance.
and volume of flowback fluids following well completions is a complex product
of the mixing of connate water and completion fluid and fluid-rock interactions
that includes precipitation and dissolution of minerals, ion exchange,
imbibition, and diffusion/osmosis. In
general, the chemistry and volume of flowback waters from Montney completions
varies with the completion program, reservoir lithofacies, depth of burial, and
hence geographically and stratigraphically.
In detail; however, the flowback volume and chemistry varies with a
plethora of variables most of which have multicollinearity. These variables include, completion fluid
chemistry, number of stages, shut-in time, surface area of the fracture
network/stimulated reservoir volume, length of flowback period, connate water
chemistry, and ambient stress field.
volume of flowback fluid from Montney completions ranges from about 15% to 30%
of the volume injected. The proportion
of connate water in the flowback water, based on conserved element and isotope
analyses, varies from about 10% to 60%, and the proportion of connate water
increases with time of flowback. The
total dissolved solids (TDS) of Montney flowback fluids range up to 230 000
mg/L, with Cl and Na ions accounting for about 75% to 95% of the total
dissolved solids. Other major ions are
Ca, K, Mg, Sr, and locally SO4.
With cumulative flowback, the TDS and most ions, for all wells,
increases linearly, although the rate of increase varies between wells, and
with stratigraphy, lithofacies (parasequence), and geographic area. Deviation from the linear increase in TDS and
conserved elements with cumulative flowback, reflects opening or closing of the
fracture system(s) with declining pore pressure, variation in connate water
chemistry and reservoir geology along laterals, and/or fractures that have
grown out of zone. Geochemical modeling also indicates that the ions that
deviate from the linear mixing model are impacted by fluid-rock interactions
including precipitation, dissolution, and/or ion exchange reactions. Reservoir surveillance using geochemical
models coupled with analysis of the flowback and produced fluids provide a
means of predicting and mitigating against salting and scaling in the
reservoir, due to dehydration of saline connate water during gas production.
of mixing between completion fluid and connate water is complex and poorly
understood. Analysis of connate water
and fluid saturations indicate that most of the unconventional Montney
Formation is below irreducible water saturation. Yet the isotopic data indicates that a
significant proportion of the flowback is connate water, even though the total
volume of water recovered is generally much less than 30% of the total volume
injected. Imbibition experiments and
measures of wettability indicate the Montney has mixed wettability, but is
preferentially oil wet. The spontaneous
and forced imbibition/osmosis of drilling and completion fluids results in
significant fracture skin damage, resulting in a decreased relative matrix
permeability by up to two orders of magnitude.
In addition, the imbibed completion fluid, depending on composition, may
weaken and ‘soften’ the fracture face promoting proppant embedment, early
collapse of non-propped fractures, and creation of fines, which in turn may
plug the proppant pack and stabilise emulsions.
large proportion of completion fluid remaining in the reservoir after flowback
is a product of the low initial reservoir water saturation, the increase in
capillary pressure of imbibed completion fluids due to fluid-rock interactions,
and much lower differential pressure during flowback than during completions.
R. Marc Bustin is a Professor
in the Department of Earth and Ocean Sciences at the University of British
Columbia and president of RMB Earth Science Consultants. Bustin received his
BSc and M.Sc. degrees from the University of Calgary and his PhD from the
University of British Columbia.
Bustin is an elected Fellow of the Royal Society of Canada
and a registered professional geologist in
the province of British Columbia.