Methodology for selecting candidates for well stimulation

Treating a well is a logical process that requires a number of previous phases before achieving the desired results.

This process begins with the evaluation of stimulation technologies and/or engineering in the field, to design the best option when it comes to increasing the productivity of a well with high skin. The basic structure of a stimulation work consists of the following phases:

  • Selection of candidates and identification of the problem of low productivity: in this stage, the best candidate for stimulation is selected. During this stage, the best treatment for a given type of “damage” is also determined.
  • Fluid selection: At this stage, the appropriate fluids, volumes and additives are selected.
  • Implementation: this stage focuses on the implementation of the acid treatment to the rock matrix, including divergence, preparation of a program with the volumes to be pumped, rates, etc.; additionally, a simulation of the treatment.
  • Treatment evaluation: in this stage, the results obtained with the stimulation treatment carried out are compared with the previous conditions of the well and with the results expected in the simulation carried out for the treatment.

These stages are used as a basis for the development and improvement of well stimulation software.

Selection of Candidates and identification of damage

Candidate selection

The production of a well declines for multiple reasons. This decline can be caused naturally by the characteristics of the reservoir fluids or properties of the rock matrix (fines, organic materials, etc.), by skin in the near wellbore during drilling and/or completion of the well, or due to mechanical difficulties in all completion processes.

Production by natural flow may also be low because the bottom coordinates of a well were not located where the reservoir properties are favorable, for example a low permeability sand.

All of these problems result in an additional pressure drop, thus affecting the skin factor.

The “skin” factor is dimensionless, a mathematical concept for the description of fluid flow from an “unaltered” reservoir towards the near wellbore.

This represents the additional pressure drop caused by reservoir flow resistance toward the completed sand face.

This value is a combination of effects of many parameters, including formation damage. To properly interpret the skin and then determine an appropriate action plan for its remediation, reservoir engineers must analyze each of the factors that contribute to the skin.

This analysis can result in additional opportunities to improve productivity, much like re-drilling. The key to selecting candidates will then be the analysis of various skins.

In this module, the “ideal” production of a well will refer to an expected production based on general information on the parameter conditions of an unaltered (undamaged) reservoir, such as permeability, thickness, porosity and saturation, etc

Many models can be used to calculate this production potential, from the simple application of Darcy's Law, to using the most complicated simulation tools.

The skin factor is frequently determined with a Horner plot of pressure data obtained from a pressure restoration test. For the purpose of candidate selection, the following skin components have been derived by various authors.

The actual skin caused by damage (the portion of the total skin that can be removed by treatments to the rock matrix) can be solved from Equation 1, as presented below:

(Eq. 1)


  • Stot = total skin factor (skin determined on the Horner plot).
  • Sdam = skin resulting from formation skin.
  • Sperf = skin resulting from hemispheric flow, etc.
  • Sturb = skin resulting from non-Darcian flow in the near wellbore.
  • Sdev = skin resulting from well deviation.
  • Sgravel = skin resulting from gravel pack.
  • Sperf size = skin resulting from perforating.

Basically, during the candidate selection process, the reservoir engineer compares a series of wells based on criteria of productivity improvement potential, formation damage, flow efficiency, and other parameters, and ranks the candidates.

The use of appropriate software for the technical evaluation of a stimulation is important, as it helps the engineer to discretize good candidate wells from poor ones.

For example, to complete this process, the engineer sets a goal according to a hypothetical budget: 3 stimulations, 2 hydraulic fracturing, and 3 new drillings (and not 8 acidifications!).

Skin Identification

If a well has high skin, the engineer must continue the procedure to classify the nature of the skin. In principle, formation skin is classified according to the processes or operations that caused its development. Skin mechanisms that must be considered are included in the following list:

  • Scales
  • Fluosilicates
  • Silica gel precipitation
  • Asphaltenes
  • Paraffin
  • Chemical production
  • Problems with bacteria
  • Clay swelling
  • Migration of clays and other fines
  • Solids/Plugs
  • Drilling mud
  • Emulsion blocking
  • Polymer skin
  • Salt bridges
  • Residual oil
  • Water blocking
  • Wettability changes

The type and depth of skin directly impacts the type of treatment that will be most appropriate for each well. Selecting a treatment without considering the cause of the skin formation will result in less “successful” treatments.

Fluid selection

The next stage of the design focuses on fluid selection. Generally, stimulation design software gives the engineer three options regarding fluid design:

  • An expert system
  • A geochemical simulator.
  • Información especificada por el usuario.

An expert system

Expert systems use logical rules based on engineering principles, the latest advances in laboratory research, and relationships determined through experience, guidelines, and best practices for designing treatments.

This method generates a complete suite of fluid systems, including acid selections, conditioner selections, volumes, additives for both sandstones and carbonates.

A geochemical simulator.

This simulator performs an iterative simulation, driven by a geochemical matrix based on the type of acidic fluid and the mineralogy of the formation.

This calculation is fundamentally much more rigorous, based on physics, chemistry and thermodynamics.

This method simulates acid invading the rock matrix and determines the optimal level between the power of the acid to dissolve clay components and the precipitation potential of the reaction products.

It also evaluates how the volume of acid could affect the loss of formation integrity and the amount of minerals to be dissolved during the procedure.


Once the engineer has determined the skin in the near wellbore and has designed the most effective acid treatment composition to eliminate the skin, an operational proposal must be designed for the implementation of the stimulation treatment.

Therefore the operational procedure is as important as the fluid design. The operation includes (1) the evaluation of possible divergents, (2) various implementation techniques, (3) the determination of the complete pumping program with stages, volumes and rates and (4) the simulation of the operation to optimize the process of design.

Divergents can be designed and simulated during the process and include sealants, inflatable plugs, balls, degradable particles, foams, gels, etc. Other placement techniques such as maximum pumping pressure (MAPDIR) and coiled tubing can also be designed and simulated.

In addition, the treatment interval can be designed using mechanical isolation techniques such as packers/bridges; injection packers can be evaluated.

Once the engineer has determined the fluids, divergence techniques, etc; the software will automatically generate a pumping program.

This program includes the stages and quantities of fluid, identifies the stages with the divergents to use, the downstream fluid pumping rates and gallons of nitrogen to use to lighten the fluid column if the well does not react.

The engineer can then export the program as a report and previously optimize it with the simulator.

The operational simulator simulates fluid pumping into the well and is a valuable tool for treatment design and analysis. A simulator of this type can handle the following variables:

  • A multi-stage treatment pump with Newtonian and non-Newtonian fluid systems.
  • Multilayer intervals with skin.
  • Sandstones (HF-HCl acid) and carbonates (wormholes).
  • Open hole completions, with or without gravel pack.
  • Bullheadings, simultaneous pipe and annular pumping.
  • Friction in the pipe.

The simulator also allows the engineer to answer questions such as the following:

  • Where do the fluids go when it is pumped to the bottom of the well?
  • Which intervals take the highest volume of treatment and which take the lowest volume?
  • How many feet does the acid penetrate into the formation? How much is the skin reduction?
  • What is the optimal pumping rate at work? Is the friction excessive?
  • What is the pumping rate to ensure efficient wormholing in carbonates?

Treatment evaluation

The final phase is the evaluation of the treatment system. Mathematically speaking, the engineer can only predict the behavior of the formation skin as the work is being performed (implementing Darcy's Law, for example).

After completion of the treatment, engineers can export the actual job data, generate another skin profile, and compare the conditions before and after the job.

It is always advisable to leave the well cleaning for a few days so that all stimulation fluids and possible fines that have remained in the well have completely circulated.

Subsequently, it is suggested to perform a pressure build-up test and determine the new skin value with the pressure data and a Horner plot. A qualitative measure of success is not to see the skin value directly, but rather the Dp skin, to later evaluate the flow efficiency.


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