Productivity of horizontal wells Petroleum Seminar

Contents

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Productivity of horizontal wells Petroleum Seminar

Introduction
Horizontal oil and gas well drilling has become one of
the most valuable technologies ever introduced in the
business It is an enhanced oil recovery (EOR) or gas
recovery method that is becoming more and more
popular as the price per barrel of oil Gets higher and
generally increase productivity to at least 2-3 times that of
vertical wells which our seminar will talk about and often
enables successful reservoir development. The oil or
natural gas in some types of reservoirs can be most
thoroughly recovered by horizontal drilling.

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Productivity of horizontal wells Petroleum Seminar

Purpose

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Productivity of horizontal wells Petroleum Seminar

Most of oil & gas reservoirs are much extensive in their

horizontal Dimension than in their vertical (thickness)
dimension by drilling a well which intersects such a
reservoir parallel to its plane of more extensive dimension.
• Horizontal drilling exposes significantly more
reservoir rock to the well bore than would be the case
with a conventional vertical well penetrating the
reservoir perpendicular to its plane of more extensive
dimension.

Definition and explanation

Is the process of drilling a well from the surface to a
subsurface location just above the target oil or gas
reservoir called the (kickoff point) then deviating the
wellbore from the vertical plane around a curve to
intersect the reservoir at the (entry point) with a near-
horizontal inclination and reaming within the reservoir
until the desired length.

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Productivity of horizontal wells Petroleum Seminar

Candidated reservoirs

There are many kinds of reservoir where the

potential benefits of horizontal drilling are obvious and
they are:

1) Thin reservoirs
A vertical well drilled into a thin reservoir will have a very
small contact surface (effectively limited by reservoir
thickness) with the oil-producing horizon.
A horizontal well in the same reservoir layer can have a

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Productivity of horizontal wells Petroleum Seminar

contact surface running the length of the reservoir.

2. Reservoirs with natural vertical fractures

Horizontal wells typically intersect thousands of
small vertical fractures and, if the reservoir contains them,
some very large ones. If the well trajectory has been
planned carefully these large vertical fractures can be
used to improve productivity, even when the overall
fracture density is low.
However, if a fault fracture system is misinterpreted the
result may be early water or unwanted gas production.

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Productivity of horizontal wells Petroleum Seminar

3. Reservoirs where water (and gas) coning will

develop
the flow
geometry
associated
with a
horizontal
drain helps to
reduce the
amount of
water or gas
coning in any given reservoir (as in the figure) This means

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Productivity of horizontal wells Petroleum Seminar

that the total volume of oil recovered before water or gas

breakthrough Can be increased. The only potential
obstacle to a significant increase in oil recovery rate is the
presence of zones with high vertical permeability (e.g. the
faults and fault-related fractures mentioned above).
However, with advance planning, these can be dealt with
using selective completion techniques. Horizontal wells
remove oil from a reservoir over a long producing zone at
relatively slow rates. In contrast, vertical Wells take oil
very quickly through much shorter lengths of borehole.
The flow geometry associated with horizontal Wells tends
to reduce the influence of heterogeneity along the long
drain– so increasing total production

4. Thin layered reservoirs

Oil recovery from water flooding can be improved
dramatically by injecting and producing from horizontal
wells, rather than using vertical wells in a traditional
Water flooding.

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Productivity of horizontal wells Petroleum Seminar

Horizontal vs. vertical

Horizontal wells cost more than vertical wells - so
what do they offer in return?

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Productivity of horizontal wells Petroleum Seminar

In problematic wells, for example, where there is a thin oil

column or a risk of early water or gas production, vertical
wells are usually very inefficient.
A comparison of horizontal and vertical well performance
clearly illustrates the potential benefits.
Every horizontal well in this example gives Better results
than its vertical counterpart.
Higher oil rates, coupled with greatly reduced gas-oil
ratios, have made horizontal wells the first choice for
many reservoirs.

Horizontal wells offer the following advantages

over those of vertical wells:

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Productivity of horizontal wells Petroleum Seminar

• Large volume of the reservoir can be drained by each

horizontal well.

• Higher productions from thin pay zones.

• Horizontal wells minimize water and gas zoning

problems.

• In high permeability reservoirs, where near-wellbore gas

velocities are high in vertical wells, horizontal wells can be
used to reduce near-wellbore velocities and turbulence.

• In secondary and enhanced oil recovery applications,

long horizontal injection wells provide higher injectivity
rates.

• The length of the horizontal well can provide contact

with multiple fractures and greatly improve productivity.

Steady State Flow Conditions

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Productivity of horizontal wells Petroleum Seminar

►The steady-state analytical solutions are the simplest

form of horizontal well solutions. These equations
assume steady state, i.e., pressure at any point in the
reservoir does not change with time.

►In practice, very few reservoirs operate under steady-

state conditions. In fact, most reservoirs exhibit
change in reservoir pressure over time. In spite of
this, steady-state solutions are widely used because:

 They are easy to derive analytically.

 It is fairly easy to convert steady-state results to

either transient and pseudo-steady stale results by
using concepts of expanding drainage boundary over
time and effective wellbore radius and shape factors,
respectively.

 Steady-state mathematical results can be verified

experimentally by constructing physical models in a
laboratory.

Pseudo steady state flow conditions

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Productivity of horizontal wells Petroleum Seminar

►Pseudo-steady state begins when the pressure

disturbance created by the producing well is felt at
the boundary of the well drainage area.

► In other words, when the fluid mass situated at the

drainage boundary starts moving towards the
producing well, pseudo-steady state begins.

►This pseudo-steady state is also described as semi-

steady state or depletion state. The name depletion
state is probably the most appropriate, because it tells
us that the reservoir has reached a point where the
pressure at all the reservoir boundaries and also the
average reservoir pressure will decrease over time as
more and more fluid is withdrawn from the reservoir

Horizontal -always best?

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Productivity of horizontal wells Petroleum Seminar

Amid the upsurge of horizontal drilling and the

predictions of its future dominance around the world we
face a fundamental question - are horizontal Wells always
better than vertical wells?

Numerical modeling carried out by researchers at the

University of Waterloo, Canada, investigated the general
Case. They found that in isotropic reservoirs horizontal
wells out Perform their vertical counterparts for two main
reasons:

►Borehole inclination.
►The longer contact length between borehole and
reservoir.

In cases where vertical permeability is significantly lower

than horizontal permeability, production can be reduced
to the point where vertical wells are better, For a fixed
length well, horizontal wells are less effective than vertical
wells only where kv/kh is Less than( 0.5)
Horizontal well drainage area

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Productivity of horizontal wells Petroleum Seminar

►A vertical well can drain cylindrical volume whereas a

horizontal well can drain an ellipsoid 3D we expect
horizontal well to drain larger reservoir volume than
vertical well.

►Due to longer well length, in a given time period

under similar operating conditions, a horizontal well
would drain a larger reservoir area than a vertical
well. If a vertical well drains a certain reservoir
volume (or area) in a given time, then this
information can be used to calculate a horizontal well
drainage area. A horizontal well can be looked upon
as a number of vertical wells drilled next to each
other and completed in a limited payzone thickness.

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Productivity of horizontal wells Petroleum Seminar

Assumptions:

►Each end of the horizontal well would drain a half-

circular area of radius b, with a rectangular drainage
shape of the horizontal well

►Assuming that each end of the horizontal well is

represented by a vertical well that drains an area of a
half circle with a radius of b

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Productivity of horizontal wells Petroleum Seminar

Drainage area calculation

Joshi (1991) proposed the following two methods for

calculating the drainage area of horizontal well

 Method I
Joshi proposed that the drainage area is represented by
two half circles of radius b (equivalent to a radius of a
vertical well rev) at each end and a rectangle, of
dimensions L(2b), in the center. The drainage area of the
horizontal well is given then by:

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Productivity of horizontal wells Petroleum Seminar

 Method II
Joshi assumed that the horizontal well drainage area is an
ellipse and given by:

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Productivity of horizontal wells Petroleum Seminar

Joshi noted that the two methods give different values for
the drainage area A and suggested assigning the average
value for the drainage of the horizontal well. Most of the
production rate equations require the value of the
drainage radius of the horizontal well, which is given by:

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Productivity of horizontal wells Petroleum Seminar

Example
A 480-acre lease is to be developed by using 12 vertical
wells. Assuming that each vertical well would effectively
drain 40 acres, calculate the possible number of either
1,000 or 2,000 ft long horizontal wells that will drain the
lease effectively?
Solution steps

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Productivity of horizontal wells Petroleum Seminar

Step 1. Calculate the drainage radius of the vertical well:

Step 2. Calculate the drainage area of the 1,000- and

2,000-ft-long horizontal well using Joshi’s two methods:

Method I
► For the 1,000-ft horizontal well :

►For the 2,000-ft horizontal well:

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Productivity of horizontal wells Petroleum Seminar

Method II
►For the 1,000-ft horizontal well :

►For the 2,000-ft horizontal well

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Productivity of horizontal wells Petroleum Seminar

Step 3. Averaging the values from the two methods:

► Drainage area of 1,000-ft-long well

►Drainage area of 2,000-ft-long well

Step 4. Calculate the number of 1,000-ft-long horizontal

wells:
Total number of 1,000-ft-long horizontal wells = 480/71=
7 wells
Step 5. Calculate the number of 2,000-ft-long horizontal
wells
Total number of 2,000-ft-long horizontal wells = 480/101=
5 wells

Productivity of Horizontal wells using steady

state solutions

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Productivity of horizontal wells Petroleum Seminar

The Well Productivity Index (J)

► Is defined as (q/∆P) where q is the flow rate and ∆P is
pressure drop. The units of PI are bbl/day/psi. Initially,
for simplicity, we will assume that we have constant
pressure at the reservoir boundary and at the well
center. This will give us the steady-state productivity
index. It is important to note that this assumes a
constant pressure being maintained at the drainage
radius and also at the wellbore.

►In actual reservoir operations, for primary recovery, this

pressure drop would change over time as more and
more fluid is withdrawn from the reservoir; therefore,
this constant pressure drop ∆P p will reduce overtime.

►The productivity of a horizontal well in most cases is

proportional to the permeability - length product (KL).
Reservoir engineers decide on how long the length (L) of
horizontal contact section should be based on typical
values of the KL product for a successful operation
►The longer the length, the greater are the chances to
penetrate a favorable geological property such as
vertical fractures, especially in heterogeneous
reservoirs, that are very favorable to enhance the oil
recovery by gravity drainage.
Steady State Solutions

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Productivity of horizontal wells Petroleum Seminar

The steady-state analytical solution is the simplest

solution to various horizontal well problems. The steady-
state solution requires that the pressure at any point in
the reservoir does not change with time. The flow rate
equation in a steady-state condition is represented by:

The productivity index of the horizontal well Jh can be

always obtained by dividing the flow rate Qoh by the
pressure drop ∆p There are several methods that are
designed to predict the productivity index from the fluid
and reservoir properties. Some of these methods include:
►Borisov Method
► The Giger-Reiss-Jourdan Method
► Joshi’s Method
► The Renard-Dupuy Method

Mathematical Solutions
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Productivity of horizontal wells Petroleum Seminar

Borisov’s Method
Borisov (1984) proposed the following expression for
predicting the productivity index of a horizontal well in an
isotropic reservoir, i.e., kv = kh

The Giger-Reiss-Jourdan Method

For an isotropic reservoir where the vertical
permeability kv equals the horizontal permeability kh,
Giger et al. (1984) proposed the following expression for
determining

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Productivity of horizontal wells Petroleum Seminar

To account for the reservoir anisotropy, the authors

proposed the following relationships:

With the parameter B as defined by:

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Productivity of horizontal wells Petroleum Seminar

The Renard-Dupuy Method

For an isotropic reservoir, Renard and Dupuy (1990)
proposed the following expression:

Where a is half the major axis of drainage ellipse and given

by Equation for anisotropic reservoirs, the authors
proposed the following relationship:

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Productivity of horizontal wells Petroleum Seminar

Calculate the steady state horizontal well productivity

using different methods if a typical vertical well drain 40
acres?
Solution:

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Productivity of horizontal wells Petroleum Seminar

1) Borisov method

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Productivity of horizontal wells Petroleum Seminar

2) Giger method

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Productivity of horizontal wells Petroleum Seminar

3) Joshi method

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Productivity of horizontal wells Petroleum Seminar

Notes and results

The productivity ratios for an 80 acres spacing horizontal
well and 40 acre spacing vertical well by different methods are
listed below

(Jh/Jv) by Different methods

METHODS PI (Jh/Jv) AREAL PI

STB/(PSI/DAY)

Borisov 48 3.7 0.60

Giger 50 3.8 0.63

Joshi 44 3.4 0.56

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Productivity of horizontal wells Petroleum Seminar

It is important note that the above productivity index comparison

assumes an unsimulated vertical well.

Effecting factors
Influence of reservoir height and horizontal well length:
►The influence of reservoir height on horizontal well
productivity is quite significant. The incremental gain
in reservoir contact area (L/h) in a thin reservoir is
much more than that in a thick reservoir.

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Productivity of horizontal wells Petroleum Seminar

►Joshi and the others predict that the productivity will

increase as the well length increases, the productivity
index will reach maximum value as well length
Continues to increase. However, for a certain
horizontal well length, the horizontal well
productivity will improve as the reservoir thickness
decrease in a uniform reservoir. To appear this
influence clearly, the comparison between

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Productivity of horizontal wells Petroleum Seminar

productivity ratio and reservoir height must be done

References:

► Ahmed ,T., "Reservoir engineering handbook"

► Joshi,s.d, "horizontal well technology"
► www.google.com

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Productivity of horizontal wells Petroleum Seminar

► www.horizontaldrilling.org

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