Joint Industry Project

PEA135 - Sand Production Prediction

Now running since 2002, Fractech's Sand Production Prediction consortium is refining earlier methods, utilising large Thick Wall Cylinder test samples and field calibration.

	Issues pertaining to intermediate strength formations have been examined
	Phase III will now extend the work to include both hard and soft formations

Background

This project was initiatied in 1997, to increase accuracy for sanding prediction. Various different rock mechanical testing methods can be used on reservoir core for sand production prediction, including:

	Thick Wall Cylinder testing (TWC)
	UnConfined Strength testing (UCS)
	Confined Strength Testing (CST)

All these testing methods exhibit problematic characteristics:

UCS method:
Core will exhibit a lower strength than it would in the reservoir and will be susceptible to failure along the bedding planes

CST method:
Will produce a stress strain curve from the tested reservoir core. However, it isn't clear at which point this curve will intersect with the onset of sanding

TWC method:
Onset of sanding usually occurs when the sidewall fails; producing ultra-conservative results and making it necessary to apply a sanding factor.

The TWC method was selected as the basis for this study, with the objective of determining field-calibrated sanding factors for various rock strengths.

A basis of the work programme has been the utilization of large TWC samples to determine the actual stress required to initiate sanding - so that geometrical constraints are no longer a factor.

Initial work was undertaken to determine the minimum ratio of internal diameter to external diameter, to factor out this constraint. Once determined, the onset of sanding could be comapred between normal TWC samples (conventionally 1½" outside diameter and ½" bore diameter) and larger TWC samples and the sanding factors determined.

Ongoing work is being undertaken to obtain further sanding factors for various strength sandstones and the field calibration of the results.

Areas of previous study include:

	4 outcrop rocks evaluated
	Gas storage wells evaluated and lab data compared to field for calibration

Other examples of field calibration are:

	Canadian Well (soft formation)
	Gulf of Mexico Well (strong, deep formation)
	Wells (North Sea)

 

Three Themes for Phase III

Harder rocks, Softer Rocks and building on the work from earlier phases (Extensions)


Two specific issues have been highlighted by members as being of high priority and as having immediate field relevance:

	Sand Production Prediction in stronger rocks at depth
	Validity of Prediction tool in shallow bores or in weak rocks

The emphasis of field calibration of the laboratory test data was considered a very important aspect of this Consortium and participants have expressed their enthusiasm to see this continued. There will therefore be a strong field-calibration element to all of the primary themes for Phase III.

Theme 1: Harder rocks

For the range of Sanding Factors (SF) so far determined, the lower values generally correlate with the stronger materials: those with UCS values of around 2000psi in the context of this project - typical of rock from deeper wells. More tests on harder rock will be done as below, to help provide more accurate Sand Production Prediction for deeper wells:

	Two outcrop rocks will be selected to give the appropriate UCS values
	For each type, standard Mohr-Coulomb tests will be completed to define friction-angle, cohesion and static modulus/stress function
	The TWC test-suite for OD/ID ratios will be completed

Reservoir core in stronger, deeper rocks with known sand production will be tested subject to core availability (one well already identified).

Theme II: Softer rocks

his has been identified as a key area for many operators. Specifically, for UCS values of the order of a few hundred psi, the TWC:UCS ratio is seen to increase.

Hence, for wells with UCS ~ few hundred psi, a prediction based on UCS may assert sanding likely, but based on TWC may suggest sand free. The objective for Theme III is to start to determine which strength indicator is most relevant for prediction of sanding in weak rocks.

	Improve Hardness Indentation measurements in low and medium strength material. Perform measurements in 100psi UCS rock. Hence determine TWC, UCS. Hardness ratios versus strength; Is hardness a better strength indicator under these conditions? Is the TWC artificially increased due to compaction, and is the extent of this increase determined by the arbitrary experimental set-up?
	Field calibration is particularly important for this work and FracTech will continue to obtain reservoir core under known sanding conditions. We will aim to evaluate four case studies.

Theme III: Extensions of Existing Work

A. Hardness Evaluation: Gas Storage Wells

During Phase II, a participant provided core, logs and sand free rates from several wells. Limited TWC's were performed (limited by core availability). The quality of the field data suggests it is worthwhile to improve the mechanical strength characterisation of the core. It is proposed, therefore, to run hardness suites across the core intervals.

B. Closed Perforation Tunnels

At the suggestion of an operator, tests will be run to investigate the impact of partially drilled holes on sanding. The tests will consist of:

Basic TWC Collapse Tests

Your comments / review of the programme as an interested party are sought and welcome.

TWC Testing Procedures

1. Sample Preparation:

The 1" diameter cylindrical rock samples are ground to ensure flat and parallel end-faces in accordance with International Society of Rock Mechanics (ISRM) Recommended Procedures. A co-axial borehole of diameter of 1/3" is cored out of the cylinder, this borehole being cleaned of any debris before testing. All coring is under 2%KCl or kerosene, as required. Samples are vacuum saturated in the same fluid before testing.

2. Loading of Sample:

The sample is placed in a Hoek cell and the radial stress and the axial stress are increased together until the point of failure. The borehole is kept full of fluid throughout the test. Pore pressure is prevented from building up by maintaining connection to atmosphere. The set-up enables a radial stress of up to 10000 psi to be applied.

Schematic Showing Thick Walled Cylinder Test Configuration

GDS 1: Controls sleeve/confining pressure
GDS 2: Controls pore pressure
GDS 3: Bleeds through the borehole

 

Costs

The membership fee is available on application (for Phase III)

Current Membership

This project enjoys support from Schlumberger and oil & gas companies globally