**3. Cemented versus barefoot completions**

fractures intersecting the wellbore and relative economics associated with perforating and

Engineered or enhanced geothermal systems (EGS) differ from conventional hydrothermal reservoirs in that supplementary hydraulic stimulation is required to create surface area needed for heat exchange, and to allow adequate fluid production. Historically, geothermal wells have been straight hole or inclined and usually employ barefoot completions. If hori‐ zontal drilling and hydraulic fracturing experience, refined to some extent with recent shale gas and shale oil stimulation campaigns, can be adapted for geothermal applications, it may be possible to improve the chances for successful EGS. One central issue, for horizontal, inclined, extended reach, or horizontally drilled wells, is whether there is merit in landing and cementing casing to allow discrete zones to be fractured, to isolate thief zones or low temper‐ ature zones, to allow future remediation and to facilitate generation of multiple fracture

Most experienced geothermal operators balk at cased, cemented and perforated completions. The arguments can be legitimate. There are supplementary costs associated with this comple‐ tion and the temperatures can make cementing and perforating challenging. Plugging of existing fracture systems during cementing is also proposed as a problem – which is easily overcome by the supplementary stimulation required. On the other hand, simple calculations suggest that proximal and interconnected fracture systems, natural or otherwise, are required for economic viability in all but the hottest scenarios. To effectively develop multiple fracture systems wellbore isolation seems to be a natural requirement. One legitimate possibility is diversion. The question remains how many intersected fractures can be stimulated? Another option is cementing and perforating. In this paper we undertake a comparative and realistic analysis to assess the impact of perforation skin, tortuosity associated with shear fractures intersecting the wellbore and relative economics associated with perforating and cementing

Simple calculations suggest that proximal and interconnected fracture systems, natural or otherwise, are required for economic viability in all but the hottest geothermal scenarios. Currently, geothermally derived power is associated with "natural" hydrothermal sys‐ tems. These are reasonably permeable and have equilibrated fluid circulation systems, with heat delivered by deep convection. They are characteristically naturally fractured and/or faulted, at least to some extent. Stimulation of fractured wells to enhance fracture conductivity is an opportunity for engineering massively stimulated systems – engi‐

**2. Requirements for a Successful Geothermal Well (EGS)**

**Keywords:** geothermal, perforations, openhole, multiple fractures

cementing geothermal wells.

360 Effective and Sustainable Hydraulic Fracturing

**1. Introduction**

systems.

geothermal wells.

Even barefoot completions are cased and cemented over a substantial portion of their length. For example, deep geothermal wells in Australia have the casing set below 4000 m or so and are open hole below that for a length of 500 m. Similar situations exist at Raft River in the United States. Nevertheless, cemented completions across thermally-productive zones will allow isolation and multiple zones can be stimulated. For example, the advantages of a cemented completion include:


**• Potential for tactical perforating to initiate multiple fractures in a single wellbore.** This is potentially a huge advantage. In openhole, without diversion, fracture initiation will seek out major discontinuities and ultimately only a restricted number of these will develop. Isolation of individual zones can ensure at least local initiation of multiple fractures. Preexisting fractures may be preferentially treated. Gale, 2008, [2] argued that natural fractures (healed) in certain shales opened at approximately 60% of the stress level needed to fracture the virgin formation. While this offers some potential to override the in-situ stress field, depending on the orientation of the fractures, there may be a greater chance of shortcircuiting and minimizing new development of fracture surface area for heat exchange. Casing and selective perforating can avoid or incorporate pre-existing fractures at the operator's discretion.

ln 2 22

 p

Do Perforated Completions Have Value for Engineered Geothermal Systems

(1)

363

http://dx.doi.org/10.5772/56211

*ff w*

D = í ý ê ú ç ÷ - ç ÷ ï ï î þ ê ú ë û è ø

p

ì ü ï ï é ù æ ö

2 2

In low permeability, fractured formations, this conventional vision of choking skin gives very small values and suggests small pressure drops. In reality, tortuous, near-wellbore intercon‐ nection between perforations and the wellbore can lead to pressure losses commonly ex‐

However, there are challenges with cased and cemented geothermal completions. These

**• Cost.** Cased and cemented completions certainly require additional tangible capital expenditure. One would anticipate that the potential for workover and the ability to

**• Placement Issues.** Geothermal wells present difficult completion environments. These can be made even worse (as with cost) because of the large diameter casing that is conventionally called out to accommodate pumping equipment. There may be situations when multiple,

**• Temperature Issues.** Perforating gun performance will need to be considered when temperatures become extremely high, in which case abrasive jet slotting may be preferable.

**• Cementing Natural Fractures.** Plugging of existing fracture systems during cementing is also proposed as a problem – which is easily overcome by the supplementary stimulation

**• Casing Integrity.** Corrosion, erosion and erosion corrosion could be long-term issues, particularly in high salinity or anomalous pH reservoirs. Operators must decide whether

generate multiple fractures will override this, as is the case in any cased wellbore.

smaller diameter wells are more economic than single large bore wells.

ln

é ù æ ö <sup>=</sup> ê ú ç ÷ - ç ÷ ê ú ë û è ø

*kh h*

*Q kh h <sup>p</sup> kh k w r*

*ff w*

*kw r*

m

p

*s*

*c*

*s*

Δps pressure drop due to finite wellbore contact

sc choke skin for radial convergent fracture flow.

pressed as skin. Some estimations are provided later.

where:

kf

wf

include:

required.

Q volumetric flow rate

k formation permeability

fracture permeability

μ dynamic viscosity

h fracture height

fracture width


#### Do Perforated Completions Have Value for Engineered Geothermal Systems http://dx.doi.org/10.5772/56211 363

$$\begin{split} \Delta p\_s &= \frac{\mathbb{Q}\mu}{2\pi kh} \left\{ \frac{kh}{k\_f w\_f} \left[ \ln \left( \frac{h}{2r\_w} \right) - \frac{\pi}{2} \right] \right\} \\ s\_c &= \frac{kh}{k\_f w\_f} \left[ \ln \left( \frac{h}{2r\_w} \right) - \frac{\pi}{2} \right] \end{split} \tag{1}$$

where:

**• Potential for tactical perforating to initiate multiple fractures in a single wellbore.** This is potentially a huge advantage. In openhole, without diversion, fracture initiation will seek out major discontinuities and ultimately only a restricted number of these will develop. Isolation of individual zones can ensure at least local initiation of multiple fractures. Preexisting fractures may be preferentially treated. Gale, 2008, [2] argued that natural fractures (healed) in certain shales opened at approximately 60% of the stress level needed to fracture the virgin formation. While this offers some potential to override the in-situ stress field, depending on the orientation of the fractures, there may be a greater chance of shortcircuiting and minimizing new development of fracture surface area for heat exchange. Casing and selective perforating can avoid or incorporate pre-existing fractures at the

**• Standard isolation benefits from an environmental perspective.** A primary goal of cemented casing is to provide another hydraulic barrier. In most cases this is not a consid‐ eration since standard casing programs should have been implemented above the openhole

**• Workover is legitimately possible.** This would seem to be a substantial advantage. Envisioning a dynamically changing reservoir, profile modification in the future could be

**• Hole integrity is increased.** This will possibly become more of an issue if sedimentary basins

**• Wider fractures and inhibited scaling?** Consider the restricted exit through perforations into a wellbore as fluid is produced. It will be demonstrated that these flow restrictions will be relatively small. However, they will still facilitate a back pressure in the formation adjacent to the perforations. One might anticipate that most of this pressure is lost very close to the perforations. The back pressure may inhibit scaling and may even lead to slightly wider fractures near the perforations. Perforation skin may be high but choke skin might

**• Ability to Pump Proppant?** Proppant placement may or may not be more effective through isolated perforated completions. Wider fractures may exist facilitating slurry entry. Also, if discrete zones are isolated, focused injection through perforations may cause more tension and/or shearing and may actually promote self-propping. This is only

**• More Contact Area?** For wellbores inclined at a significant angle to productive fractures in openhole the contact area between the wellbore and the fracture may be small. Cased, cemented and perforated completions may actually alleviate some contact related pressure losses. For example, Mukherjee and Economides, 1991, [3] considered skin that would develop because of inadequate contact between a vertical transverse fracture and a hori‐

operator's discretion.

362 Effective and Sustainable Hydraulic Fracturing

start to be routinely exploited.

actually be reduced.

an hypothesis.

zontal well. They expressed this choking effect as:

sections.

desirable.

Δps pressure drop due to finite wellbore contact

Q volumetric flow rate

μ dynamic viscosity

k formation permeability

h fracture height

kf fracture permeability

wf fracture width

sc choke skin for radial convergent fracture flow.

In low permeability, fractured formations, this conventional vision of choking skin gives very small values and suggests small pressure drops. In reality, tortuous, near-wellbore intercon‐ nection between perforations and the wellbore can lead to pressure losses commonly ex‐ pressed as skin. Some estimations are provided later.

However, there are challenges with cased and cemented geothermal completions. These include:


the benefits of casing, cementing and perforating in the short-term override the costs of degradation with time, and whether these completions jeopardize well or system produc‐ tivity and economics.

In a perforated completion, and to some extent openhole, stimulation effectiveness and the economics for producing adequate mass flow rates are influenced by near-wellbore completion characteristics. During stimulation, the goal is to transmit pressure to the tip of the fractures that are being created or inflated. Near-wellbore pressure drop requires additional horsepower with accompanying cost for the stimulation. Similarly, during production, the goal is to minimize frictional losses – in the fractures and especially where the fractures intersect the wellbore, near and through the perforated completion. In either case it is necessary to minimize

Do Perforated Completions Have Value for Engineered Geothermal Systems

http://dx.doi.org/10.5772/56211

365

A substantial amount of work has been done to understand pressure drop that occurs through perforations. One facet of this research has been to evaluate the pressure loss in a complicated perforation connection from the wellbore to the formation during injection. Eftaxiopoulos and Atkinson, 1996, [8] provided an elegant mathematical approximation. This built on earlier work by Yew and Li, 1988, [9] as well as Yew et al., 1989 [10]. These latter authors applied three-dimensional elasticity to assess hydraulic fracture growth from inclined wells. This is a situation that does promote complicated interconnection with the main hydraulic fracture and where a perforated completion may offer significant advantages over open hole. Initiation, propagation and linkage of fractures formed from individual perforations were considered. Yew et al., 1993, [11] continued these evaluations. A landmark practical presentation of these

In 1991, Behrmann and Elbel [13] carried out laboratory block testing to study the complex interconnection of multiple fractures growing from perforations. These authors suggested the strong potential role of a microannular fracture link. "In both cases, despite ideal laboratory conditions, clean wellbore and casing, and short cement interval, the wellbore annulus is pressurized during any pumping treatment. Therefore, in the absence of any optimally oriented defects (perforations), fractures will initiate as though the completion were openhole. Fracturing pressure will obviously be higher than in open hole, but initiation sites and extension geometry will be the same. Thus, it is theoretically possible to have different fractureinitiation sites with identical perforation orientations if two wells have substantially different

In 1995, Romero et al. [14] presented numerical evaluations related to near-wellbore injec‐ tion pressure losses attributed to communication (perforations), fractures (turning and twisting) and multiple fractures. Their interest was in mitigating high treatment pressure and unanticipated screenouts. They allocated a near-wellbore pressure loss to the sum of these three effects – perforation pressure drop, turning/twisting or tortuosity and perfora‐ tion misalignment. For the perforations themselves, they considered the perforation tun‐ nels as orifices. That relationship is (see Crump and Conway, 1988, [15], Lord et al., 1994,

near-wellbore pressure losses.

**6. Perforation skin — Pressure loss during injection**

concepts was provided by Weng et al., 1993 [12].

wellbore damage."

[16] Shah et al., 1996 [17]):

**• Pressure Losses.** For commercial purposes, single well rates are high. It is often argued that pressure losses will be too extreme. Simple calculations to follow explore some of these mechanisms, and suggest that this may or may not be the case.

Pending successful isolation (casing and cementing), it is still necessary to complete the well. What are the methods for carrying this out? Perforating is the first logical choice. Abrasive jetting is also a possibility and may in fact ultimately turn out to be preferable. Alternatively, diversion is advocated as a methodology for isolation in openhole – and there is good logic for this, if the diverter can tolerate incremental pressures between fracturing events. Diversion could be considered in open or cased hole scenarios to maximize fracture contacts with the wellbore.
