1. Aspects of poorly consolidated formations1..1 sedimentation and consolidated formations1..2 petrophysics1..3 mechanical properties

2. Conventional hydraulic fracturing modeling2..1 model components2..2 Fracture mechanics fundamentals2..3 Hydraulic fracture propagation model2..4 Comparison of fracture propagation models

1) ASPECTS OF POORLY CONSOLIDATED FORMATION

The definition of consolidate formations can be interpreted as the formations with depositional

environment that favor all the process required to its drilling, exploration, production, fracturing, among

others. The poorly consolidated formations are most common than the people think; one of the most

important formations of this kind exist in the Gulf of Mexico. The poorly consolidated formations have

to be handled with more care because the petrophysical and mechanical characteristics of these

formations makes the normal operation a harder procedures; and these procedures imply more

investigation, resources and of course, money. The first part of this work is about aspects related with

depositional environment, petrophysical and mechanical characteristics of poorly consolidated

formations.

1.1 SEDIMENTATION AND CONSOLIDATED FORMATION

The poorly consolidated formation have direct relationship with highly depositional environments, this

term refers to the environments in which enough energy was to be present to transport large materials

such as rocks and boulders, on the other hand a low energy depositional environment is only able to

transport materials such a fine sands; The first one occurs in a relatively short time span, so the process

to obtain the formation are not well done so the characteristics of the obtained formation are not equal

to the Consolidated formation. The most common transport methods to produce poorly consolidated

formation corresponds to deltaic or turbiditic depositional setting. This two environments have in

common the facility of how they can move sediments and the fast time spin buried. Also they can

transport considerable amounts of coarse material and very find sediments. They are not the only

scenarios in which the poorly consolidated formation can be developed, but the next process

description correspond to them.

The find sediments can be in suspension in marine waters to then flocculate and settle as lumps in deep

water; in turn they create a blanket on top of the sandstone deposit, generating impermeable layers

after burial.

The rapid burial and the presence of the impermeable layers, may effectively trap the fluid and the

particles into sealed deposit. This fluid will be ejected later in the subsequent burial and compaction.

Rapid burial, along with the presence of overlain fine sediments (i.e. low permeability layers), may

effectively trap the fluid and particles into a ‘sealed’ deposit. As a result, fluid ejection is constrained

during subsequent burial and compaction. For this deposit (Grains and fluid), the overburden is

supported by both of them (grains and fluid). The relation is expressed as:

Sv=σ v+Pp

Where:

Sv = Overburdenσ v =Effective StressPp =Pore Pressure

To understand the influences of the solid part (grains) and the fluids, is define that the effective stress is

supported by the solid part and the pore pressure is supported by the fluid inside the pore.

As was described before in this work, in sealed deposits the fluid cannot be ejected upon burial, so the

overburden is mainly supported by the trapped fluid, and then the pressure in the solid part (grains) is

lower, hence the compaction in this deposits cannot be compared with a normal compacted basin.

The lithification is directly affected for the pressure between the contact points, so this pressure

reductions is going to be reflected in the speed of the lithification. Now the deposits are capable to

contain hydrocarbons, but they can start to produce them before the cementation and lithification

process ends completely.

The lithification process slows down, due to the reduction of pressure at the contact points, which

ultimately leads to formations that are capable to contain hydrocarbons, but that may be put in

production when the cementation/lithification process is still incomplete. This changes in the normal

process of cementation and lithification can be seen in the tangential contacts between the grains. The

contacts between grains are classified as tangential, long, convex and sutured, and the classification

depends directly of the effective stress.

1.2 PETROPHYSICS.

The diversity, different characteristics, sub-environments and variability is something really common

into the poorly consolidated rocks. This is one of the consequences of the depositional environments

described before.

The deltaic environment present different types of structures as a consequence of waves, tides and

strength of the main river currents. The sediments deposited along the proximal margin would tend to

be of better quality than those located toward distal margins because the coarser sediments are

deposited at the proximal margin. The types of architectural elements presented in the turbiditic

environment correspond to sheets, channels and levees. That’s why a single structure may have

different characteristics.

Although the characteristics that define a poorly consolidated formation are too much, these kind of

formation present mostly very good quality.

The different books and papers about the Gulf of Mexico corroborate that the theory of poorly

consolidated formation is completely true and also compare the characteristics of this formation with

the poorly consolidated formation around the world to conclude that they exhibit, in average, excellent

quality, with porosity in a range of 30’s and also good permeability, associated with a sand size particles

arranged in moderate-sorted structure. All these properties are the result of the quick burial of the

sediments prevented comprehensive compaction and lithification of sediments; in other words the

mechanical compaction and the cement generation still occur, but to a lesser degree. All these

characteristics make the poorly consolidated formation really susceptible to ambient stress changes

caused by reservoir depletion.

References

Hydraulic Fracturing of Soft Formations in the Gulf Coast”. SPE Formation Damage Control Symposium,

Trends in Shallow Sediment Pore Pressure – Deepwater Gulf of Mexico”.

Stress-Dependent Permeability: Characterization and Modeling”. SPE Paper No. 56813 e.g.

Rock Mechanics for Industry; Amadei, Kranz, Scott & Smeallie (eds.)

Trends in Shallow Sediment Pore Pressure – Deepwater Gulf of Mexico. Ostermeier, R.M., Pelletier, J.H., Winker, C.D., and Nicholson, J.W.

Hydraulic Fracturing Stimulation in Poorly Consolidated Sand: Mechanisms and Consequences. Mohamad Khodaverdian, SPE, and Paul McElfresh, SPE, Baker Oil Tools. SPE 63233

Fundamentals on Physical Geology. Seerepat Jain