computational investigation of hydro-mechanical effects on ...
computational investigation of hydro-mechanical effects on ...
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casing shoe, but within<br />
the reservoir, to stimulate<br />
through hydraulic and chemical methods [2].<br />
Hydraulic stimulati<strong>on</strong> carried out in Desert Peak welll<br />
27-15 from<br />
September 2010 through April 2011 led<br />
to a nearly<br />
60-fold increase in injectivity [2]. This<br />
stimulati<strong>on</strong><br />
was carried<br />
under two different fluid<br />
pressure c<strong>on</strong>diti<strong>on</strong>s relative to the least principal<br />
stress. An initial period <str<strong>on</strong>g>of</str<strong>on</strong>g> shear stimulati<strong>on</strong>, whichh<br />
increased injectivity by<br />
more than <strong>on</strong>e order <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
magnitude, from ~0.011 to ~0.15 gpm/psi, was<br />
carried out<br />
in a series <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
steps at low fluid pressuress<br />
up to 4.5 MPa well head pressure (WHP). This<br />
maximum WHP was chosen to remain below the<br />
magnitude <str<strong>on</strong>g>of</str<strong>on</strong>g> the least horiz<strong>on</strong>tal principal stresss<br />
(WHP ~5.2<br />
MPa), as measured in this well just below<br />
the casing shoe by a mini-hydraulic<br />
fracturing testt<br />
[15]. This<br />
low-flow-rate phase was<br />
immediately<br />
followed by a large-volume c<strong>on</strong>trolled hydraulic<br />
fracturing operati<strong>on</strong> thatt lasted more than 23 days,<br />
which was<br />
carried out at high injecti<strong>on</strong> rates and<br />
WHP in excess <str<strong>on</strong>g>of</str<strong>on</strong>g> the least principal stress. This<br />
hydraulic fracturing stage resulted in an<br />
additi<strong>on</strong>al 4-<br />
fold increase in injectivity [2]. Temperature-Pressure--<br />
exited and<br />
stimulated well 27-15 at two primary locati<strong>on</strong>s: 1) the<br />
bottom <str<strong>on</strong>g>of</str<strong>on</strong>g> the open-hole secti<strong>on</strong> during<br />
the low-flow--<br />
rate injecti<strong>on</strong> phase and 2) the hydraulic fracture just<br />
below the<br />
casing shoe<br />
during the high-flow-rate<br />
injecti<strong>on</strong> phase.<br />
During the<br />
EGS experiment, a total <str<strong>on</strong>g>of</str<strong>on</strong>g> 42 micro-<br />
earthquakes (MEQs) with magnitudes<br />
ranging from<br />
+0.10 to +0.74 were recorded between<br />
EGS well 27-<br />
15 and injecti<strong>on</strong>/producti<strong>on</strong> wells to the south-<br />
southwest, including in proximity to injecti<strong>on</strong> wells<br />
21-2 and 22-22 (see Figure 2 and Figure 3) [2]. Alll<br />
but <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> these MEQs occurred<br />
during the<br />
c<strong>on</strong>trolled hydraulic fracturing stimulati<strong>on</strong>, with <strong>on</strong>ly<br />
<strong>on</strong>e event (discussed below) occurring<br />
during shear<br />
stimulati<strong>on</strong>. During all stimulati<strong>on</strong><br />
stages, the<br />
greatest injectivity gains are associated with the<br />
initiati<strong>on</strong> or occurrence <str<strong>on</strong>g>of</str<strong>on</strong>g> these MEQs under either<br />
c<strong>on</strong>stant or<br />
decreasing wellhead pressure (Figure 1) .<br />
Spinner logs show that the injected fluid<br />
This suggests that shear failure (i.e., faulting)<br />
resulting in<br />
the generati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> MEQs is a key physical<br />
process c<strong>on</strong>trolling the evoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> transmissivity.<br />
Variati<strong>on</strong>s in injecti<strong>on</strong> rate occurred in wells 21-22<br />
and 22-222 at various<br />
times during the EGS<br />
stimulati<strong>on</strong>, especially prior to the c<strong>on</strong>trolled<br />
hydraulic fracturing stage. In some cases, this makes<br />
it difficult to establish a unique correlati<strong>on</strong> between<br />
EGS operati<strong>on</strong>s and the observed seismicity.<br />
However, no significant<br />
variati<strong>on</strong>s in injecti<strong>on</strong> rate<br />
were occurring in wells 21-2 and 22-222 when the first<br />
MEQ was observed <strong>on</strong> Sept 17, 2010, during the low-<br />
flow-rate shear stimulati<strong>on</strong> (Figure 1). As discussedd<br />
below, this<br />
is <strong>on</strong>e reas<strong>on</strong> this stage <str<strong>on</strong>g>of</str<strong>on</strong>g> the Desert Peak<br />
EGS stimulati<strong>on</strong> was selected for analysis in the<br />
present paper.<br />
Figure 1: Low-flow rate injecti<strong>on</strong> phase, Sept 2010: well<br />
27-155 well-head pressure (WHP) and injecti<strong>on</strong> rate. The<br />
observed Sept 17 micro-earthquakee (MEQ) occurs after<br />
aboutt 4 days <str<strong>on</strong>g>of</str<strong>on</strong>g> injecti<strong>on</strong> and it is followed by a remarkable<br />
increase in the injecti<strong>on</strong> rate under c<strong>on</strong>stant wellhead<br />
pressure (Figure from<br />
[2]).<br />
Poor r focal sphere coverage and limited c<strong>on</strong>straints <strong>on</strong><br />
the seismic velocity model make it difficult to: (1)<br />
derive the exact source mechanism for these MEQs,<br />
(2) detect events smaller than magnitude M w < +0.1<br />
or (3) define the locati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> individual events with<br />
precisi<strong>on</strong>. Nevertheless, tensilee failure produces<br />
relatively high frequency signals at the crack<br />
tip –<br />
typically <str<strong>on</strong>g>of</str<strong>on</strong>g> M