Abstract
Horizontal open hole wellbores in carbonate formations have become an everyday
occurrence to improve production economics. Carbonate formations are typically
stimulated by using hydrochloric acid. An acid treatment in the well
configuration described provides a significant challenge in diversion to
accomplish complete stimulation of the lateral.
Various techniques have been employed to overcome this problem. These include
pre-perforated or slotted liners, specialty treating fluids, solid diverting
materials and tools. All have had some successes in various regions of the
world.
Presented are three case histories in which open hole packers and sliding
sleeves have been employed with and without the aid of specialty treating
fluids. These cases cover Devonian TVD 3,170 m (10,400 ft), Devonian TVD 2,682
m (8,800 ft) and San Andres TVD 1,737 m (5,700 ft) formations. Details of these
treatments, as well as production responses, are discussed.
Introduction
Stimulation of carbonate reservoirs is typically used to restore or enhance
production to an economic level. Acid, whether organic or inorganic in nature,
is a natural means of effecting such stimulation of these types of
lithologies(1-8). Acid fracturing is the most widely used technique
for deep stimulation of limestone or dolomite formations. Matrix treatments,
whether at a minor injection rate (low permeability reservoirs) or at high
rates (high permeability reservoirs), are also commonly used to effect a
shallow damage bypass. San Andres and Devonian Formations are common carbonate
reservoirs producing in the Permian Basin with lithologies, depths and
bottomhole temperatures that vary significantly. In addition, horizontal
completions are becoming routine in some areas where these formations are being
produced.
Geologic and Reservoir Descriptions
Devonian
Microscopic examination of core samples in the area of the treated wells
revealed a mixture of the following lithologies: chert, cherty limestone, limey
chert, dolomitic cherty limestone and dolomitic limey chert(9-11).
Microporosity, rather than interparticle porosity, has been created by partial
substitution of the original limestone matrix by silica (chert). The
post-depositional processes included karst weathering, cave wall deposition and
collapsed cave material accumulating as brecciated chert zones. Figure 1
illustrates the variance of the composition through the Devonian in the wells
at 2,682 m (8,800 ft) TVD.
The Devonian is a solution gas drive reservoir with some water production.
Original bottomhole pressure was found to be approximately 38,611 kPa (5,600
psig) from a DST in 1986. Bottomhole temperature is approximately 57 °C (135
°F). Permeability varied from less than 0.01 mD to greater than 16 mD, with an
average of approximately 2 mD. Porosity varies from less than 1% to greater
than 13%, with an average of approximately 11%.
San Andres
The San Andres ( ∼1,737 m, 5,700 ft) is a Dolomitic Formation with solution gas
drive in combination with gas cap expansion(9, 12). Average
permeability is over 9 mD with an average porosity greater than 13%. Acid
solubility varies from 78 to 92% in 15% hydrochloric acid. The main components
of the lithology are Dolomite (77 to 92%) and Anhydrite (3 to 20%). Bottomhole
temperature is typically around 38 °C (100 °F). These values are illustrated in
Figure 2.
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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History
- Original manuscript received:
28 March 2007
- Meeting paper published:
12 June 2007
- Revised manuscript received:
13 April 2009
- Manuscript approved:
4 May 2009
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