Drill Stem Test (DST): Qualitative DST Chart

When drilling has reached total depth and the formation to be evaluated has been determined by samples, logs, and cores, a Drill Stem Test may be ordered.

The testing equipment is attached to the end of the drill string. A hydraulic valve system keeps the string dry as it is being lowered into the wellbore. The testing equipment also incorporates a sealing device or packer that effects against the wellbore.

When testing equipment has reached the prospective formation, the formation is isolated from the rest of the wellbore by the packers. A valve opens and the formation is allowed to produce into the dry drill pipe.

At this time, a graphic pressure versus time chart of flow performance is recorded. This chart is produced by mechanical recorders and/or electronic pressure/temperature gauges. The data derived from the Drill Stem Test (DST) give a computation of the formation’s permeability, damage ratio, productivity index, transmissibility and radius of investigation.

After the operator determines that a formation test is to be conducted, the test tool is assembled and lowered into the wellbore. A graphic chart is obtained from two recorders.

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What is a Drill Stem Test (DST) and what data is attainable?

Drill stem test (DST) is the conventional method of formation testing and reservoir evaluation which obtains reservoir data under dynamic (rather than static) conditions. A DST is essentially a temporary completion, a method of evaluating reservoir formations without costly and time-consuming completion procedures.

The basic drill stem test tool consists of a packer or packers, valves or ports that may be opened and closed from the surface, and two or more pressure-recording devices. The tool is lowered on the drill string to the zone to be tested.  The packer or packers are set to isolate the zone from the drilling fluid column.

The valves or ports are then opened to allow for formation flow while the recorders chart static pressures.  A sampling chamber traps clean formation fluids at the end of the test.

Analysis of the pressure charts is an important part of formation testing.

Data attainable from a Drill Stem Test

Normal data recovery from a Drill Stem Test includes items such as fluid recovery and description, blow descriptions test times, mud and hole data and the pressure/time data as recovered from the chart record. These items are reported from the field and recorded on a field data sheet or envelope.

In addition to field data (direct information), additional reservoir characteristics may be calculated utilizing the test data recovered in the field (indirect information). Some of these reservoir characteristics are:

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Major Reference Guides for Well Integrity

In this article, the major reference guides for Well Integrity are presented. These guides can be considered as reference documents as well as a basis for training classes on the subject of Well Integrity.

The standard defines the minimum functional and performance oriented requirements and guidelines for well design, planning and execution of safe well operations. The focal of the standard is well integrity.

Well integrity is defined to be “application of technical, operational and organizational solutions to reduce risk of uncontrolled release of formation fluids throughout the life cycle of a well”. The standard focuses on establishing well barriers by use of WBE’s (well barrier elements), their acceptance criteria, their use and monitoring of integrity during their life cycle. The standard also covers well integrity management and personnel competence requirements. The standard does not contain any well or rig equipment specifications.”

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Back Pressure Regulators for Gassy Sucker Rod Lifted Wells

The problem of heading (flow off and pump) is often encountered in gassy wells. This heading effect which can blow the tubing dry occurs as follows:

  • Gas expansion in the tubing as oil from the reservoir travels towards the surface (due to gas pressure decrease).
  • Formation of a gas “plunger” that can push the liquid above it out of the tubing and into the flow line at high speed. As the gas forces the liquids out of the tubing, the pressure in the tubing decreases rapidly and the gas expands even more.
  • This heading behavior of reservoir fluids causes cycles of high production followed by low or no production.

When heading process starts, the expanding gas pushes the liquid into the flowlines and increases production for a short time. In the meantime, the liquid leaving the tubing is replaced by more and more free gas. Eventually, the tubing is blown dry and production stops until the tubing fills with liquid again.

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Introduction to Well Integrity

According to Norsok D-010, well integrity is defined as the “application of technical, operational and organizational solutions to reduce the risk of uncontrolled release of formation fluids and well fluids throughout the life cycle of a well”.

Well Integrity is defined in ISO/TS 16530-2 as: “containment and the prevention of the escape of fluids (i.e. liquids or gases) to subterranean formations or surface”

In API RP 65-2, well integrity is defined as: “a quality or condition of a well being structurally sound with competent pressure seals by application of technical, operational, and organizational solutions that reduce the risk of uncontrolled release of formation fluids throughout the well life cycle “.

Following from the aforementioned definitions of well integrity, the personnel planning the drilling and completion of wells will have to identify the solutions that give safe well life cycle designs that meet the minimum requirements of the standard.

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