Presented at Short Course on Geothermal Development and Geothermal Wells,

organized by UNU-GTP and LaGeo, in Santa Tecla, El Salvador, March 11-17, 2012.

GEOTHERMAL TRAINING PROGRAMME LaGeo S.A. de C.V.

AERATED DRILLING

Sal Antonio Ramos

Perforadora Santa Brbara S.A. de C.V.
Prolongacin Boulevard Sur y 15 Av. Sur, Colonia Utila
Santa Tecla, La Libertad
EL SALVADOR
sramos@psb.com.sv

ABSTRACT

The technique of aerated drilling in the geothermal wells of El Salvador has solved
a series of obstacles that occurred previously in drilling, well completion, and
intervention. The last drilling stage in commercial production wells and reinjection
wells is carried out with a fluid mixture made up of water-air or water-air-foam.
The main objective of this operation is to achieve return of circulation to the
surface, preventing the accumulation of the cuttings in the production or reservoir
zones. This brief study is based on the drilling and completion of well CHI-3A in
the Chinameca Geothermal Field in 2009. It presents in detail the experiences
encountered during operation, where air was primarily been used as the drilling
fluid, and then to stimulate the wells, and lastly, to obtain early production
characteristics before having the final evaluation. This was the first well where
Perforadora Santa Brbara provided this service to LaGeo.

1. INTRODUCTION

1.1 Aerated drilling

Drilling with aerated fluid is a technique used in drilling of geothermal wells, mostly in the last stage,
which it is carried out in search of zones of interest, where partial and/or total loss of circulation occur
and where the production reservoir (permeable zone) is located.

In El Salvador, the methodology for the final well drilling stage, whether it is for production of fluids
or for residual fluid reinjection, is presented as follows:

The first stage starts with a water- based drilling fluid with low viscosity and density. This fluid is
prepared with the following materials such as lignites, lignosulfonates, polymers such as sodium
carboxymethyl cellulose and partially hydrolyzed polyacryl amide (PHPA). Due to the low viscosity
of this drilling fluid, not all of the rock cuttings can go up to the surface. Therefore, 60 to 70 s/L
Marsh high-viscosity pills are prepared, which are pumped to the bottom the well. The fluid rises up
through the annular space, carrying all the cuttings with it, thereby ensuring the well free of these
materials. This technique is called a borehole cleaning.

As drilling proceeds, the drilling fluid is gradually lost once the zone of interest is reached until total
loss of circulation is encountered. As a result, the cuttings no longer rise to the surface and this
becomes a serious problem as they may accumulate in the zones of interest (reservoir zones for a
Ramos 2 Aerated drilling

production well) and high permeability zones for a reinjection well). The well, production or re-
injection well, can decrease its efficiency when connected to the Geothermal Plant System.

Aerated drilling or aerated fluid drilling has provided a solution to the aforementioned problems,
which is also known as the "underbalanced drilling", using a drilling fluid mixture of water-air or
water-air-foam.

The advantages of this technique are presented in the following:

a) Fluid return is achieved, thereby transporting the cuttings to the surface;

b) The guarantee of a thorough cleaning of the well and of the zones of interest (reservoir and
permeable zones);
c) Higher drilling speed;
d) Diminished damage to the formation;
e) Reduced density of the drilling fluid and consequently reduced hydrostatic pressure, which
lowers the probability of the drill string to become stuck due to differential pressure;
f) Decreased cooling of the reservoir due to the use of water for drilling;
g) Identification of the changes in formations with the corresponding depths by having the
cuttings brought to the surface, which aids in establishing the stratigraphy; and
h) Faster response upon finishing the drilling operation; and discharging the well for its
evaluation.

This paper is a summary of the experiences during the 8" drilling stage (the last stage) of well CHI-
3A in the Chinameca Geothermal Field.

2. AERATED DRILLING IN WELL CHI-3A

2.1 Background

Well CHI-3A was drilled directionally on the 8" stage. From the 9 5/8" casing shoe drilling was
carried out from 1080 m up to 1248 m depths, with a low viscosity and low density fluid, which was
prepared with water base, lignites, lignosulphonates and polymers, such as sodium carboxymethyl
cellulose and partially hydrolyzed polyacrylic amide.

The first loss of circulation was encountered at 1248 m depth, from 10 to 20 m3/h. Drilling continued
up to 1258 m depth with partial loss of circulation from 30 to 60 m3/h and continued up to 1267 m
depth, where total loss of circulation occurred.

It should be emphasized that the drilling fluid became diluted gradually until it completely
transformed into water at a depth of 1267 m. The final depth of this well was 1720 m.

2.2 Objective of aerated fluid drilling

At 1267 m depth , preparations were made for aerated fluid drilling in order to obtain circulation, with
the objective of keeping the well clean, preventing the drill string from becoming stuck due to
differential pressure, obtaining cuttings on the surface to be analyzed for the construction of the
stratigraphy of the well, drilling with higher rate of penetration, and minimizing the cooling of the well
as the drilling fluid is a water-air-foam mixture.

2.3 Description of the equipment used in the aerated drilling for well CHI-3A

A description of the equipment used in the aerated drilling for well CHI-3A is given in Table 1.
Aerated drilling 3 Ramos

TABLE 1: Description of the equipment used in the aerated drilling for well CHI-3A

Qty EQUIPMENT DESCRIPTION

3 Compressors Air compressor, Ingersoll Rand, Model XHP 1170 scfm to 350 psi,
Rotary screw, After cooler, CAT Engine diesel, Model C-15 ACERT,
540 BHP to 2100 RPM .

1 Booster Air Booster, Gardner Denver JY-500, Joy WB12, 2475 scfm to 2500
psi, and 3200 scfm to 800 psi. Two stage 5.5 x 7 and 3.5 x 7,
Piston stroke 7, with after cooler, CAT Engine diesel, Model C-18
ACERT, 630 BHP to 2100 RPM.

Mist pump Triplex Pump, plunger 13/4, stroke 41/2, 49 GPM to 2500 psi, Brand
1 (Injection pump) West Texas, Model HP-100-M, with engine diesel, Deutz, model BF-
4L914 97 BHP to 2300 RPM. Two tanks, 20 bbl/tank, open top.

2 Texsteam pump Plunger size 11/4, work pressure 2000 psi, 500 gallons/day, DI Buna-n
(Injection pump) packing. For injecting chemical materials: Foaming agent, corrosion
inhibitor agent, oxygen scavenger agent and other chemical products.

1 Rotating head Rotating Head 135/8 Assembly, Brand Washington, dressed with:
Viton seal and O-rings, consisting 135/8-3000 Air bowl complete with
clamp, 10-2000 outlet, Lubrication system, HD 51/4 Hexagonal drive
bushing, Stripper rubber wrench, 4 BU Butyl Stripper rubber.

1 Air / Fluid Vertical Air Fluid Separator (Figure 1), type Keystone, capacity of
separator separation: Air 2400 scfm and Liquid 1200 gpm to 20 psi. Outside
Diameter 61.2 and Length 5.2 meters.

1 Flowmeter Daniel Simplex Orifice Plate Holder, Type of fitting welding neck end,
(Daniel) ANSI CLASS 1500, 707C-TSC, 3705 psi work pressure, Sch 80,
Orifice plate 2 x 1/8 model 500, universal type and Teflon seal ring.

1 Flow recorder Barton Flow Charts Recorder, model 202E, 9A Recorder-DP-OD,

(Barton) Differential Pressure 0.0 to 100 water column, Static Pressure 0.0 to
2000 psi, Safe Working Pressure 2500 psi.

1 Choke manifold Choke Manifold with Choke Valve Adjustable, 2 DN, LP, FIG. 602 M
x F, Work pressure 6000 psi, Ball Valves 2 x 6000 psi work pressure
and Daniel Flowmeter 2 DN, 1.939 ID, Sch 80, ANSI 1500.

High pressure Pup Joint, Chiksan Hose and flexible Hose 2x 12 feet, FIG 602 M x F,
line and work pressure 6000. Swivel Joint 2, Style 10 and Style 50, M x F,
connections work pressure 6000 psi. Swing check valve flapper 2, work pressure
3000 psi, Ball valve 2 x 6000 psi work pressure.

1 Temperature Borehole return fluids temperature register from 0.0 350 C.

register

As can be observed in the layout of the air drilling equipment used in the last stage of 8" in well
CHI-3A (Figure 2), the primary rotary screw compressors are of low pressure, discharging the air flow
at a pressure of 350 psi. Depending on the air flow requirements, one, two or all of the three
Ramos 4 Aerated drilling

compressors can be operated, with the total flow at the injection manifold. The air is then driven
through a flexible hose, 3" in diameter and at a pressure of 1000 psi, until it reaches the other high
pressure piston compressor, called BOOSTER (Figure 3), which operates at a maximum pressure of
2500 psi. The air is cooled at the booster output and is led through the 2" x 6000 psi high pressure line
towards the choke manifold, which is comprised of the choke valve, the flowmeter and the Barton
recorder. The choke valve is then regulated depending on the air flow needs for the drilling operation,
where a certain amount of air flow is discharged to the atmosphere and the rest passes through the
flow meter connected to Barton Recorder; the injection pressure is given in psi and the differential
pressure in inches of water. With the two pressure values, the air temperature at the booster output
and the compressibility factor of air, together with the design features of the flowmeter, these data are
then entered into an "Orifice Flow Calculator" software, provided by the company that designed the
"DANIEL MEASUREMENT AND CONTROL" flowmeter, and thus obtaining the air flow rate that
is being injected into the well that is being drilled.

Flange 8

VMS1 Flange 8

Air Line

Fluid Line to Shale Shaker

Second Line, fluid

outlet

Flange 8

VMS2

Flange 8
VMS3
VMS4

Flange 6
Flange 8
By-Pass Line

Fluid Line to Shale Shaker Flange 8

Clean and Drain Line 2

FIGURE 1: Air/fluid separator installed in air drilling system, well CHI-3A

Aerated drilling 5 Ramos

BOOSTER WB-12 COMPRESOR #1 COMPRESOR #2 COMPRESOR #3

BLEED OFF ATMOSFERA

CHEMICAL

TEX STEAM

VBBA
VCT1

VBP4
VB6

VBP3
VAT1

VBC1 VBC2 VBC3

VBBB VBBI

VCB1
VC1 VB5 VBP2 VBP1 CKP1
VB7

VBM2
VBM1 VBM3
VC2

VBMP1
Leyenda Descripcin Smbolo Descripcin
VBMP5
VBMP4

TANK #1
VBC Vlvula de Bola del Compresor IR 2 Vlvula Bola Fig. 602
VBMP3
VBM Vlvula de Bola Manifold 3 Aire 2 Vlvula Check Fig. 602
VCMP1

TANK #3 VCB Vlvula Check del Booster 3 Vlvula Bola Fig. 200
VBMP2 PUMP
VBBA Vlvula de Bola Booster A (Entrada Aire) 2x 3 Mts. Manguera Fig 602
DEUTZ
VBBB Vlvula Bola Booster B (By-pass Booster) 3 x3 mts. Manguera Fig 200
MIST PUMP
VBBI Vlvula Bola Booster I (Servicio Aire) 2 Tee Fig. 602

CKP Choke Vlve Patn de Inyeccin Adaptador P/ Tex Steam

TO STAND

VBP Vlvula de Bola patn Inyeccin. Manguera LP BARTON

PIPE

VB Vlvula Bola Lnea Inyeccin. 2 Choke Fig. 602

PROYECTO PERFORADORA SANTA BARBARA VC Vlvula Check. Manguera HP BARTON

VC3

VAT Vlvula Aguja H.P. Tex Steam. BARTON RECORDER TEMP/PRESION

VCT Vlvula Check H.P. Tex Steam

VB8

Vlvula Aguja 5000 psi

VBMP Vlvula de Bola Mist Pump.
TITULO: GENERAL INJECTION LINE Vlvula Check 5000 psi
VCMP Vlvula Check Mist Pump.
Pozo: CHI-3A 2 Meter Run
Manifold 3 P/Toma Aire.
Fecha: 15/Agosto/09 RIG UP COMPRESISON EQUIPMENT Manifold 3 P/Toma Aire.

FIGURE 2: Aerated drilling equipment layout in well CHI-3A

FIGURE 3: Complete diagram for aerated fluid drilling

Ramos 6 Aerated drilling

2.4 Description of the operations with aerated fluid in well CHI-3A

On August 13th, 2009, when the well encountered total loss, circulation was undertaken for cleaning
and observation of the well at 1267 m depth. Viscous pills with polymers were pumped to do a
borehole cleaning and maintenance of the drill bit. A lubricating pill was then pumped and sent to the
bottom of the well. The SINGLE SHOT survey was then performed at 1252 m, with the following
results of Inclination 29.75 and Azimuth 116.50.

Drilling was resumed with water until it encountered a total loss of circulation at 1277 m depth. A
complete cycle of circulation was carried out and the 81/2 drill bit was recovered to the surface to
modify the drill string.

The hydrostatic level was measured in the well, the result of which was 690 m. The drill string with a
diamond crown bit was connected to cut the core sample from 1277 m to 1279.05 m depths by only
using water at total loss and a rate of 436 gpm.

After having cut the core sample, a new 81/2 diameter drill bit was connected and was lowered to the
95/8 casing shoe . The accessories for aerated drilling were then connected with the 135/8 API 3000
Rotating Head and the 4" Stripper Rubber.

The safety meeting was held with all involved personnel; among them was the Superintendent of the
Compressed Air Equipment who briefed everyone on the details, precautions, dangers, and possible
accidents that could occur when carrying out aerated drilling. The 81/2 drill bit was then lowered to a
depth of 1277 m pumping water in total loss of circulation at a rate of 262.0 gpm.

The drill string was retrieved up to the 95/8 casing shoe (1088 m) to start the aerated fluid operations.
Circulation was carried out with a 262 gpm water flow rate and a 700 cfm air flow rate for 2.5 hours
without obtaining any return to the surface. Afterwards, it was lowered to 1200 m using the same
water and air flow ratio with the purpose of decreasing the hydrostatic column. The 81/2 drill bit was
lowered to 1322 m. Circulation was carried out for two hours, with a water flow rate of 349 gpm and
an air flow rate of 1700 cfm , presenting a 57 bar pressure without obtaining any return to the surface.
A foaming agent, with concentration of ranging from 0.05% to 0.20% V/V, was added to the water-air
mixture to obtain return of circulation to the surface (Figure 4).

Drilling was performed until reaching 1358 m depth with the same water-air flow rates and the same
foam concentration, observing return of circulation to the surface. Circulation was carried out for an
hour to completely clean the well.

Drilling was continued up to a depth of 1444.5 m with the water-air-foam mixture, a water flow rate
of 240 gpm, air flow rate of 1700 cfm, foaming agent at 0.18% V/V and a pressure of 50 bars, and
with return of circulation to the surface. Thus, circulation was carried out for 1.5 hours to clean the
well. Afterwards, a lubricating pill was pumped and sent to the bottom of the well. The SINGLE
SHOT survey was performed at 1430 m depth, yielding the following results of Inclination 32.7 y
Azimuth 114.50

Drilling of the 81/2 stage was completed on August 31st, 2009 at a depth of 1720 m, with water flow
rates ranging from 250 to 350 gpm, air flow rates from 700 to 1700 cfm, a foaming agent with a
concentration between 0.05% and 0.20% V/V, and pumping pressures between 45 and 50 bars, with
return of circulation to the surface.
Aerated drilling 7 Ramos

FIGURE 4: Return of the mixture water-air-foam-cuttings on the surface

REFERENCES

Ramos, S.A., 2009: Final air drilling report of well CHI-3A, Chinameca geothermal field.

Daniel, 2008: Fundamentals of orifice meter measurement and control.

Keystone Drill Services Inc., 2008: Compressors and booster equipment.

Washington Rotating Control Head Inc., 2008: Drilling rotating head.