Direct Methods for Rock & Reservoir Property Evaluation

Direct Methods in Formation Evaluation

Direct methods are primary techniques in formation evaluation used to obtain crucial rock and reservoir properties. Properties such as permeability, porosity, and water saturation are typically determined through these direct measurements. Generally, direct methods are performed using three main approaches: drilling operation logs (including mud logging), coring, and productivity tests.

Drilling Operation Log (Mud Logging)

• Provides a continuous record of the wellbore as it is being drilled.
• Involves the examination of drill cuttings and mud returns to determine rock and reservoir properties of the formation.
• Drilling parameters and observations are plotted against depth for comprehensive analysis.

Coring Techniques

Coring involves retrieving physical samples of the formation and its pore fluids from the subsurface to the surface. These samples are then transported to a laboratory for detailed analysis to determine properties such as porosity, permeability, water saturation, capillary pressure (P_c), and other critical reservoir characteristics. There are several types of coring methods, including conventional coring, diamond coring, wireline coring, sidewall coring, and special coring.

Conventional Coring

• A traditional method used to retrieve core samples from the bottom of the wellbore.
• Typically employed in consolidated formations.
• Requires the removal of the entire drill string and core barrel assembly from the hole to retrieve the core samples.
• Utilizes a specialized core bit, coated with sharp diamonds, which slowly cuts through the formation, even the hardest rock, instead of a standard drill bit.
• Core samples typically range from 3-4 inches in diameter and are usually up to 30 feet long.

Sidewall Coring Techniques

Sidewall coring is a technique where core samples are obtained from the wall of a previously drilled zone. This method typically involves firing hollow bullets into the formation, which then retrieve the sample attached to a steel cable. Sidewall core samples usually have a diameter of approximately 1 inch and a length ranging from 1 to 2 inches.

Two general types of sidewall coring are Percussion Sidewall Coring and Rotary Sidewall Coring.

Percussion Sidewall Coring

• Primarily suitable for soft, unconsolidated formations.

Advantages

• Speed and low cost (significantly cheaper than conventional coring).

Disadvantages

• Tends to alter the formation by shattering harder rock, especially in percussion sidewall coring.
• Recovery rates in hard rock formations are typically very low.

Rotary Sidewall Coring

• Capable of recovering sidewall cores without the shattering impact associated with percussion systems.
• Utilizes a core drill to obtain sidewall core samples.
• Can retrieve a higher number of samples, typically around 30 cores per run.