NAME OF COURSE:

PET 604

TARGETED AUDIENCE:

Undergraduate/Postgraduate Student(s) and lecturers of Petroleum

Engineering Department

COURSE OBJECTIVE:

The objective of this presentation is to clarify the potential and

production of gas in tight sands.

STUDY METHOD:

Seminar

NAME OF COURSE LECTURER:

Dr. Ekejiuba

NAME OF INSTITUTION:

Federal University of Technology (FUTO), Owerri

CLASS:

2013/2014 ACADEMIC SESSION

i

TABLE OF CONTENT

SECTION 1.0:

Introduction

SECTION 2.0:

Literature Review

2.1 Definitions of Tight Gas Sand Reservoirs

2.2 The Resource Triangle

2.3 Case Study 1

2.4 Case Study 2

SECTION 3.0:

Reserve Estimation and Applicable methods

3.1 Reservoir Characterization

3.2 Reserve Estimation and Applicable methods

SECTION 4.0:

Conclusion

SECTION 5.0

References

ii

TABLE OF FIGURES

FIG 1:

Illustrates the principles of the resource triangle in different forms

FIG 2:

U.S tight gas sand basins

FIG 3:

Resource triangle for tight gas in United States

FIG 4:

World natural gas reserves by area

FIG 5:

Natural gas reserves for six selected countries

FIG 6:

Proven natural gas reserves around the world

FIG 7:

Decline curve – Rate Vs Time: Exponential, Harmonic and Hyperbolic

iii

TABLES

TABLE 1:

Reserves Estimation Comparison of Conventional Gas Reservoirs and Tight Gas

Sand Reservoirs

iv

v

Section 1.0:

1.1 INTRODUCTION

Tight gas sands have significant gas reserves, which require cost-effective well

completion technology and reservoir development plans for viable commercial

exploitation. Tight sand gas is referred to as gas that is stuck in a very tight

formation underground, trapped in uncommonly low permeability hard rock, or in

a sandstone formation in most cases, however they could also be found

carbonates such as limestone that is unusually impermeable and non-porous

(tight sand). Typically, these formations contain net pay zone ranging from 25 to

over 250 feet, original reservoir pressure from 1500 to 15,000 psi and porosity

from 3 to 10 percent.

Tight gas is the term commonly used to refer to low permeability reservoirs that

produce mainly dry natural gas. Tight gas sand reservoirs are generally referred to

as unconventional gas reservoirs. “Unconventional” in the case of hydrocarbon

energy is a term used to define those resources that are not easily accessible and

can only be produced at a higher cost than those other resources that are

considered “conventional”.

Tight gas sand reservoirs like other unconventional reservoirs (e.g. carbonates,

coal bed methane, limestone and shale) have one thing in common—a vertical

well drilled and completed into a tight gas sand (e.g. sandstone) reservoir must be

successfully stimulated to produce at commercial gas flow rates and produce

commercial gas volumes. Normally, a large hydraulic fracture treatment is

required to produce gas economically from tight gas sand reservoirs. In some

naturally fractured tight gas sandstone reservoirs, horizontal wells and or

multilateral wells can be used to provide the stimulation required for

commerciality.

To optimize the development of a tight gas sand reservoir, the geoscientists and

engineers must optimize the number of wells drilled, as well as the drilling and

completion procedures for each well. Often, more data and more engineering

Man-power are required to understand and develop tight gas reservoirs than are

required for higher permeability, conventional reservoirs. On an individual well

basis, a well completed in a tight gas sand reservoir will produce less gas over a

longer period of time than one expects from a well completed in a higher

permeability, conventional gas reservoir. As such, many more wells (or smaller

well spacing) must be drilled in a tight gas reservoir to recover a large percentage

of the original gas in place (OGIP), when compared to a conventional reservoir.

It is clear that unconventional resources are contributing increasingly and in a fast

rate to our energy supply, therefore the future of our energy supply lies

essentially on unconventional resources among which is a gas from tight sands.

The affordability of unconventional resources is conditioned by how cost effective

its development and its extraction are. It is important to develop adequate

extraction methods and techniques to effectively produce gas from tight sand

reservoirs.

Planning the development of the field is one of the most important steps in the

extraction process after geological, geophysical and petro-physical study of the

field have been executed. Nowadays numerical simulators have become handy

tools to accomplish such purpose. Conventionally, a lot of wells must be drilled to

get most of the gas out of these tight formations.

The best definition of tight gas reservoir is “reservoirs that cannot be produced at

economic flow rates or recover economic volumes of natural gas unless the well is

stimulated by a large hydraulic fracture treatment or produced by use of a

horizontal wellbore or multi-lateral wellbores.”

Section 2.0:

LITERATURE REVIEW

2.1 DEFINITION OF TIGHT GAS

In the 1970s, the United States government decided that the definition of a tight

gas reservoir is one in which the expected value of permeability to gas flow would

be less than 0.1 md. This definition was a political definition that has been used to

determine which wells would receive federal and/or state tax credits for

producing gas from tight reservoirs. Actually, the definition of a tight gas reservoir

is a function of many factors, each relating to Darcy's law.

.................... (eq. 1)

The main problem with tight gas reservoirs is that they do not produce at

economic flow rates unless they are stimulated—normally by a large hydraulic

fracture treatment. Eq.1 illustrates the main factors controlling flow rate. Eq.1

clearly shows that the flow rate, q, is a function of permeability (k); net pay

thickness h; average reservoir pressure (p¯); flowing pressure (pwf); fluid

properties (β¯μ¯) drainage area re; wellbore radius (rw); and skin factor (s). Thus,

to choose a single value of permeability to define "tight gas" is not wise. In deep,

high pressure, thick reservoirs, excellent completions can be achieved when the

formation permeability to gas is in the microdarcy range (0.001 md). In shallow,

low pressure, thin reservoirs, permeabilities of several millidarcies, might be

required to produce the gas at economic flow rates, even after a successful

fracture treatment.

The reservoir cannot be produced at economic flow rates or recover economic

volumes of natural gas unless a special technique is used to stimulate production.

Specifically, large hydraulic fracture treatments, a horizontal well-bore or

multilateral wellbores must be used to stimulate flow rates and increase the

recovery efficiency in the reservoir.

So what is a typical tight gas reservoir? There are no "typical" tight gas reservoirs.

They can be:

Deep or shallow

High pressure or low pressure

High temperature or low temperature

Blanket or lenticular

Homogeneous or naturally fractured and

Single layered or multilayered.

The optimum drilling, completion and stimulation methods for each well

are a function of the reservoir characteristics and the economic situation.

Some tight gas reservoirs are in south Texas, while others are in the deserts

of Egypt. The costs to drill, complete and stimulate the wells, plus the gas

price and the gas market affect how tight gas reservoirs are developed. As

with all engineering problems, the technology used is a function of the

economic conditions surrounding the project.

2.2 THE RESOURCE TRIANGLE

Discussion on gas in tight sands is not complete without mentioning “The concept

of the resource triangle” as was used by Masters and Grey to find a large gas field

and build a company in the 1970s.The concept is that all natural resources are

distributed log-normally in nature. If you are prospecting for gold, silver, iron,

zinc, oil, natural gas, or any resource, you will find that the best or highest-grade

deposits are small in size and, once found, are easy to extract. The hard part is

finding these pure veins of gold or high permeability gas fields. Once you find the

high-grade deposit, producing the resource is rather easy and straightforward. To

best illustrate the difference between conventional and unconventional

resources, the natural gas resource triangle shown below in different forms was

devised based on the concept developed by Masters and Grey in the 1970’s.

Fig. 1: illustrates the principle of the resource triangle in different forms.

As you go deeper into the gas resource triangle, the reservoirs are lower grade,

which usually means the reservoir permeability is decreasing. These low

permeability reservoirs, however, are much larger in size than the higher quality

reservoirs. The scale on the right side of the figure also illustrates typical values of

formation permeability for tight gas sands or carbonates. Other low quality

resources, such as coal-bed methane, gas shales, and gas hydrates would likely

have different permeability scales. Easily accessible resources are at the top of

the triangle and are small in quantity as compared to unconventional resources

which are available in large quantities but very challenging with respect to

exploration and production. Unconventional gas provides over half of the US gas

production

The common theme is that low quality deposits of natural gas require improved

technology and adequate gas prices before they can be developed and produced

economically. However, the size of the deposits can be very large when compared

to conventional or high quality reservoirs. The concept of the resource triangle

applies to every hydrocarbon-producing basin in the world. One should be able to

estimate the volumes of oil and gas trapped in low quality reservoirs in a specific

basin by knowing the volumes of oil and gas that exist in the higher quality

reservoirs.

2.3 CASE STUDY 1

TIGHT GAS IN THE UNITED STATES

Since the 1950s, the oil and gas industry has been completing and fracture

treating low permeability wells in the United States. However, it was the natural-

gas price increase in the 1970s that spurred significant activity in low permeability

gas reservoirs. Since the 1970s, sustained increases in natural gas prices, along

with advances in evaluation, completion and stimulation technology, have led to

substantial development of low quality gas reservoirs. Below is a map showing

the location of the major tight gas basins in the United States.

FIG 2: U.S. Tight Gas Sand Basins

The estimates of gas production, reserves, and potential from the tight gas basins

in the United States are compatible with the concept of the resource triangle.

Below is another illustration showing tight gas resource base estimates from the

Gas Technology Institute (GTI). The gas produced through the year 2000 from

tight gas reservoirs is estimated to be 58 Tcf. Proven reserves in tight gas

reservoirs are 34 Tcf. Thus, the sum of produced gas plus proven reserves adds up

to 92 Tcf. GTI estimates the volume of technically recoverable gas from known

U.S. tight gas accumulations at 185 Tcf. The term "technically recoverable" means

that the gas is known to exist; the technology is available to drill, complete,

stimulate and produce this gas; but the gas cannot be booked as reserves until

the wells are drilled and the reservoirs are developed. The next category is called

undiscovered, which represents the GTI estimate of gas that is likely to be

discovered in known tight gas basins. Finally, the largest category is called

resources. This value represents the gas in place in the U.S. tight gas basins.

Substantial improvements in technology or changes in the gas market are

required before the gas in the resources category can be produced economically.

FIG 3: Resource triangle for tight gas in the United States.

Figure 4 below further illustrates world natural gas reserves by area. These

estimates are available to everyone from the BP website: www.bp.com. Notice

that most of the gas is in eastern Europe, the former Soviet Union, and the Middle

East. Figure 5 below also shows the gas reserves for six selected countries. Russia

has 1,700 Tcf of gas reserves, while Iran has 812 Tcf. Notice that the United States

has only 167 Tcf of proven gas reserves, of which 34 Tcf are from tight gas

reservoirs. The last bar on the graph shows the sum of the estimates of

technically recoverable tight gas and undiscovered tight gas in the United States

as estimated by GTI. Summing all three categories of tight gas (proven, technically

recoverable, and undiscovered), one could expect that 569 Tcf of gas will be

produced in the future from tight gas reservoirs in the United States, which is

substantially more than the 133 Tcf (167–34) of proven gas reserves that are

currently booked for conventional gas reservoirs.

FIG 4: World natural gas reserves by area

FIG 5: Natural gas reserves for six selected countries

FIG 6: Proven Natural Gas Reserves around the world.

2.4 CASE STUDY 2

TIGHT GAS OUTSIDE THE UNITED STATES

The purpose for discussing tight gas in the United States in such detail are to provide statistics to validate the resource triangle concept and to provide information on how important tight gas sand production currently is to the United States. The next logical question is to ask, "Can we extrapolate what we know about tight gas in the United States to the other oil and gas basins around the world?" The answer is yes. The resource triangle concept is valid for all natural resources in all basins in the world, so it is logical to believe that enormous volumes of gas in unconventional reservoirs will be found, developed, and produced in every basin that now produces significant volumes of gas from conventional reservoirs. Unfortunately, no organization has published a

comprehensive review and estimate of the volume of gas that might be found in tight reservoirs around the world. In fact, the volume of gas in conventional reservoirs around the world is still being revised upward as exploration for natural gas increases.

If we use the concept of the resource triangle, the volume of gas-in-place in tight reservoirs could be orders of magnitude higher than the volume of gas known to exist in conventional reservoirs, in every basin. The information in Fig. 4 shows that the current estimate of world gas reserves is about 5,250 Tcf. By comparing the ratio of current conventional gas reserves in the United States (133 Tcf) to the potential for gas production from tight reservoirs in the United States (569 Tcf), one could envision that eventually 20,000+ Tcf of gas will be produced from tight reservoirs around the world, given proper economic conditions and technology improvements. Without question, interest in tight gas sand reservoirs around the world increased substantially during the 1990s. In many countries, tight gas is defined by flow rate and not by permeability. Development activities and production of gas from tight reservoirs in Canada, Australia, Mexico, Venezuela, Argentina, Indonesia, China, Russia, Egypt, and Saudi Arabia have occurred during the past decade. Large hydraulic fracture treatments are being used more commonly around the world to stimulate gas flow from low permeability reservoirs. Such activity will only increase during the coming decades.

Section 3.0:

GAS IN TIGHT SAND RESERVE ESTIMATION AND APPLICABLE

METHODS

3.1 RESERVOIR CHARACTERIZATION

One of the particularities of tight gas sand reservoir is the versatility of its

characteristics as such; in the characterization of the reservoir one must consider

the following:

Geology: This defining regional thermal gradients, the regional pressure

gradients as well as the stratigraphy of the region.

Reservoir Continuity: This affects particularly the characteristics of the

drainage area, and the orientation of hydraulic fractures as it is conditioned

by horizontal stresses in all of the reservoir layers. Reservoir continuity

depends essentially on regional tectonics.

Reservoir data acquisition: This is done in two ways, and the most

important and the most economical being the openhole well logging that

helps determine the volumetric (porosity, saturation), and the petro-

physical (resistivity, density) properties of the reservoir, some cases may

include special logs such wellbore image and nuclear magnetic resonance.

The second type of data acquisition is coring, this provides essentially fluid

flow properties and mechanical properties of the rock

Mechanical Properties: Most tight gas reservoir must be stimulated

before it is economically produced; the most popular method is hydraulic

fractures. For such procedure to be successful one must be aware the

mechanical properties of the pay zone and its surroundings, these

properties include: in-situ stress, Young’s modulus and Poisson’s ratio.

Permeability Distribution: This is an important concept to be considered

when it comes to forecasting gas flow. Holditch determined that most tight

gas reservoir follow the similar log normal permeability distribution

pattern. Therefore, the median permeability value is the best

approximation for central tendency as opposed to the arithmetic mean

values which tend to overestimate permeability values.

3.2 RESERVE ESTIMATION AND APPLICABLE METHODS

Estimating reserves in tight gas sand reservoir is a delicate task as conventional

well known methods such as volumetric method, and material balance method

rarely apply due to assumptions used in developing these methods, Table 1 below

elaborates on each and their range of application. The most common methods as

far as tight gas sand reservoirs are concerned are:

Curve analysis (decline and type) and

Reservoir models: When simulators are available.

Decline curve analyses are readily available and less cumbersome than others.

Method

Conventional Gas

Reservoir

Conventional Gas Reservoir

Tight Gas Sand Reservoirs

Volumetric Accurate in blanket reservoirs Used only when n wells have been

drilled

Material

Balance

Accurate in depletion drive

reservoirs

Should never be used

Decline

Curves

Exponential Decline usually

accurate

Must use Hyperbolic Decline

Reservoirs

Models

Used to simulate the field Used to simulate individual wells

Table 1: Reserve Estimate Comparison of Conventional Gas Reservoir and Tight Gas Sand

(Holditch, 2006)

Declines curve analysis is based on:

Production history

Uses plots of flow rate vs. time and

Cumulative production (Cartesian or log-log scale): To determine reservoir

parameters, reserves and predict future production.

Arps in the 1940’s determined that production rate decline behaviors were

similar to one of the hyperbolic family of curves.

Depending on the curvature, decline behavior can be group as follow:

Exponential

Harmonic and

Hyperbolic.

These behaviors are illustrated in the figure below:

FIG 6: Decline Curve - Rate vs. Time - exponential, harmonic, hyperbolic

Tight gas reservoirs decline predominantly as hyperbolic decline type and are

analyzed with semi-log plot of production rate vs. time and obey to the following

relationships below:

Section 4.0:

CONCLUSIONS

After elaborating on the background and the evolution of unconventional

resources, it is clear that unconventional resources are contributing increasingly

and in a fast rate to our energy supply. Therefore, the future of our energy supply

lies essentially on an unconventional resource among which is tight sand gas. The

affordability of unconventional resources is conditioned by how cost effective its

development and its extraction are. It is important to develop adequate

extraction methods and techniques to effectively produce tight sand gas

reservoir.

Planning the development of the field is one of the most important steps in the

extraction process after geological, geophysical and petro-physical study of the

field have been executed. Nowadays numerical simulators have become handy

tools to accomplish such purpose.

Conventionally, a lot of wells must be drilled to get most of the gas out of these

tight gas sand formations.

Section 5.0:

REFERENCES

Thesis on Production Optimization of a tight sandstone gas

reservoir by Cyrille W.D.

Petroleum Engineering Handbook (Chapter 7- Tight Gas

Reservoirs) by Larry W.L.

Tight Gas Reservoirs: An unconditional Natural Energy Source

for the Future, Article by Naik G.C.

Distinguished Author Series: Tight Gas Sand

By Stephen .A. Holditch