Int Field St.indd - GRC Geothermal Library

Int Field St.indd - GRC Geothermal Library

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Geothermal Resources Council Transactions, Vol. 28, August 29 - September 1, 2004
Production Problems Review of Las Tres Virgenes Geothermal Field, Mexico
Juan De Dios Ocampo-Díaz1 and Mario Rojas-Bribiesca2
1
Universidad Autónoma de Baja California, Facultad de Ingeniería, Mexicali, B. C., México
Comisión Federal de Electricidad, Residencia General de Las Tres Vírgenes, B. C, Sur, México
2
Keywords
and wells has CO2 as the predominant gas. Problems occurred
in production wells due to high mud losses during drilling
and to calcite scaling and plugging, both of which have led to
decline in overall well productivity. This paper analyzes and
discusses these problems that have affected production in “Las
Tres Vírgenes” geothermal wells.
Lost circulation, mud damage, decline production, calcite
scaling.
ABSTRACT
“Las Tres Vírgenes” geothermal field is located in the Santa
Rosalia Basin, Baja California State, Mexico.. The geothermal
system is related to a quaternary volcanic complex, composed
of three volcanoes. The reservoir is liquid dominated and
highly influenced by fractured rocks and associated storage of
fluid at high temperatures. Comision Federal de Electricidad
began exploration in 1982 and currently of the nine wells that
have been drilled, 6 are producers and 3 are reinjectors, with
depths between 1290 m to 2500 m. The produced water has
a sodium-chloride composition characteristic of geothermal
brine completely equilibrated at a temperature estimated at
280°C. The chemical composition of gases from fumaroles
Introduction
“Las Tres Vírgenes” geothermal field is located in the eastern
coast of the Baja California Peninsula, 35 km NW of Santa
Rosalia city (Figure 1). The Comision Federal de Electricidad
(CFE) started geothermal exploration activities in 1982, with
geophysical, geological and geochemical feasibility studies
(Ballina and Herrera; 1984; Lira et al., 1984; Quijano, 1984;
Tello, 1988; Gutiérrez-Negrín, 1990; Viggiano, 1992; LópezHernández et al., 1995; Tello, 1997). These studies supported
the decision to drillthe first exploratory well, LV-2, in 1986.
(Lopez, 1998). Additional studies were
completed to achieve a better understanding of the natural state of the
geothermal system (Tello, 1998; Vargas
and Garduno, 1988; Bigurra, 1989;
Lopez et al., 1989). In a second stage
of exploration, studies of the structural
geology of the hydrothermal system
were completed to optimize the locations of new wells to be drilled (Tovar,
1989; Gutierrez, 1990, Viggiano, 1992;
Garcia y Gonzalez, 1998).
Objectives
The main objective of this paper is to present the
problems that affect the production characteristics of “Las
Tres Vírgenes” geothermal field. The original causes and
consequences are analyzed and possible alternatives are
discussed that may remedy or minimize the production
problems observed in wells of this geothermal reservoir.
Figure 1. The Las Tres Vírgenes Geothermal Field.
499
Ocampo-Díaz and Rojas-Bribiesca
Geological Setting
Table 2. Production Data of “Las Tres Vírgenes” geothermal Wells.
Well
Within a regional context, the geothermal system of LTV
is located in the Santa Rosalía basin, a NW-SE trending PlioQuaternary depression which forms the western limit of a
deformation zone related to the Gulf of California opening
(Demant, 1981; López-Hernández et al., 1995). The western
border of the basin is occupied by a fault system, also trending NW-SE. Gutierrez-Negrín (1990) and Viggiano (1992)
established that “Las Tres Virgenes” is located in a tectonically
active area associated with the faulting process of the Gulf of
California opening.
Three Quaternary volcanic centers have been identified
(from oldest to the youngest): La Reforma caldera, the Sierra
Aguajito and Las Tres Vírgenes complex. The chemical compositions of these volcanic complexes are calco-alkaline , with the
exception of an alkaline rich pyroclastic flow and some basaltic
cones observed at La Reforma, which are of peralkaline type
(Sawlan, 1986). The most active thermal zone has been located
in the northern limit of the youngest volcanic center whose age
is around 0.44 Ma. This complex comprises of three volcanoes:
La Virgen, El Azufre and El Viejo. In the north zone of Las Tres
Vírgenes complex the volcanics are of dacitic composition. The
south zone shows a different chemical composition covering a
range from basaltic to rhyolitic products. Regional and local
geological maps of the LTV geothermal system have already
been published (López-Hernández et al., 1995). A complete compilation of the main geological and geophysical features of “Las
Tres Virgenes” geothermal field is reported by López-Hernández
et al. (1995). The geothermal field’s heat source appears to be
related to the magma chamber of the La Virgen volcanos.
LV-1
LV-2
LV-3
LV-4
LV-5
LV-7
LV-8
LV-11
LV-13
Ws (t/h)
Steam
43
-8.0
13
7.0
-33
12
Depth
m
1887
1291
2150
2500
1859
1925
1715
2081
2500
Present status
Not Flowing
Not flowing
No flowing.
Flowing
Not Flowing
Not flowing
Flowing
Flowing
Not flowing
Geochemical Aspects
Geochemical studies for determining chemical and isotopic
composition of geothermal fluids were completed by Tello
(1997). Fluid geothermometry based on the chemical analyses
is reported by Tello (1997) and Na/K geothermometer temperatures (Verma and Santoyo, 1997) are shown in Table 1.
Figure 2. Las Tres Vírgenes Geothermal Well locations.
Figure 2 presents a simplified geological map showing the
location of the wells drilled in “Las Tres Vírgenes” geothermal
field. By July 2001, the first electrical power units, two 5-MWe
condensing turbines, were installed at “Las Tres Vírgenes”.
Steam production during 2002 was 0.28 million metric tons, at
an average annual rate of 37 t/h. On average, during the 2003
year, three production and one injection wells were in operation. Only one power unit operated between January and the
first days of November, with a total generation of 19 GWh.
Electricity produced from the field has been powering nearby
towns, which are isolated from Mexico’s national electrical grid
(Residencia de “Las Tres Virgenes”, 2003)
Table 1. Geothermometer Temperatures.
Well
LV-1
LV-3
LV-4
LV-5
Temperature (Na/K)
263 ± 25 °C
263 ± 25 °C
269 ± 26 °C
263 ± 25 °C
Carbon dioxide is the predominant gas phase (> 80 %).
Hydrogen and sulphuric acid are the other gases detected in
significant quantities. The water type is alkali chloride (sodiumchloride) and is in equilibrium with the rock.
Problems in Production Wells
Production and Generating History
Two problems are affecting the production of wells: i) calcite scaling both inside the well and the reservoir (Wells LV-3,
LV-4, LV-11 and LV-13) and ii) mud losses while drilling. Both
problems are causing low initial flow rates and rapid declines
in well productivity. All wells drilled in the geothermal field
Currently of the 9 wells that have been drilled, 6 are producers and 3 are reinjectors (Quijano et al. 2003). Table 2 shows the
production characteristics reported by Cardenas et al. (1998)
and Residencia de “Las Tres Virgenes” (2004).
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Ocampo-Díaz and Rojas-Bribiesca
Well LV-11
have encountered partial and total circulation mud loss zones
as a consequence of the intense fracturing characteritic of the
field. For example, during drilling of well LV-613, the viscous
mud amount lost was about 5,583 m3 (35,000 bbls), increasing
to about 18,571 m3 (117,000 bbls) after several attempts to fish
a portion of the drill bit and stabilizer that was stuck near
the bottom of the well (Jaimes et al. 2003). It is believed that
mud lost to the formation is causing a skin effect around the
wellbore, a problem that was confirmed after acidizing wells
LV11 and LV-13 at the end of 2002 (Jaimes et al. 2003).
This well was drilled during September 2000 and reached
a total depth of 2081 m. During drilling activities several partial and total loss circulation zones were encountered, mainly
in the open hole section. Jaimes et al. (2003) reported a total
volume of viscous mud loss of 5,61 m3. As a consequence of
the low production during the first production period and
the fast declining steam flow rate , CFE engineers decide to
acidify the well in an attempt to improve the permeability .
For the 18 months prior to acidifying the well the steam flow
rate averaged about 12 t/h. After acidification the steam flow
rate increased to 33 t/h. Caliper logs run in the well after the
acidifying did not indicated a severe scaling problem inside
the casing, but did indicate formation damage by mud lost
had been reduced.
Scaling Problems
One of the most common production problems in geothermal fields is calcite (calcium carbonate) scale deposition.
Calcite blockages formed in the wellbore decrease significantly
the output of production wells. Calcite scaling is experienced
in almost all the geothermal fields around the world, i.e. in
the Dixie Valley geothermal field, Nevada (Benoit, 1989), in
Ohaaki geothermal field, New Zealand (Clotworthy et al., 1995
and Nogara, 1999), in Seltjarnarnes geothermal field, Iceland
(Kristmansdottir et al., 1995) and in Coso geothermal area
in California (Evanoff et al., 1995). In an extreme case, most
of the production wells and surface facilities in the Kizildere
geothermal field in Turkey were blocked by calcite scale and
serious generation losses were incurred (Durak et al., 1993).
Todaka et al., (1995) also reported calcite deposition, together
with anhydrite (calcium sulphate) in the wellbores in Oguni
geothermal field in Kyushu, Japan, and BacMan geothermal
field in Philippines.
Like other geothermal fields, Las Tres Vírgenes geothermal
production field is not an exception with regards to the problem
of calcite scaling. Calcite is one of the few minerals whose
solubility decreases with increasing temperature; usually the
calcite deposition mechanism occurs through CO2 liberation
when the brine begins to boil.
Well LV-13
This well was drilled at the end of 1993, with a total viscous mud loss during drilling reported by Jaimes et al. (2003)
of about 18,771 m3 (32,236 bbls) - three times more than
the volume of mud lost in well LV-11. The damage caused in
the feedzones (permeable production intervals) resulted in a
clear production decline to about 2 to 3 t/h, mainly over the
last 10 months. Caliper logs run prior to acidifying showed
an obstruction at 2200 m. The production results achieved
after acidifying showed a reduction in formation damage,
and an increase in steam flow rate to 13 t/h. Production data
analysis showed that the fluids were boiling in the reservoir,
CFE personnel are planning to repeat acidification in this
well during 2004.
Concluding Remarks
“Las Tres Vírgenes” geothermal field is a reservoir located
in the basement with low permeability and pressure conditions.
All wells drilled in this geothermal reservoir have experienced
high viscous mud losses while drilling, which caused damage
to the formation. Additional problems during production is
calcite scaling seen both in the wells and reservoir zones
CFE engineers have applied some strategies to minimize
the damage by mud loss during the drilling and improve the
production by acidification. It is necessary to continue analyzing and test new techniques and methods to achieve mproved
production in this complicate geothermal field.
Well LV-3
This well was drilled at the end of 2001, the production in
this well started after the perforation. During 1999 this well
had been flowing through several orifice plate diameters, and
it showed a clear decline production tendency, about 20% of
the initial values. The production declines increased during
2000 until the well stopped flowing at the end of this year.
The chemical analysis of scaling samples collected from the
well during workover activities, using an x-ray diffraction, are
included in Table 3.
References
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Table 3. Well LV-3, X ray diffraction analysis.
X-ray diffractions analysis
Magnetite
Calcite (CaCO3)
Cuarzo ( SiO2) and Cristobaita (SiO2)
Halita (NaCl)
Silvita (KCl)
Others
% weight
5
50
27
ND
ND
11
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