Improved Core Recovery in Laminated Sand and Shale Sequences
Authors: Bradburn F.R. , Cheatham C.A. 
Published: December 2007

Introduction

Coring and core analysis are essential to the exploration, development, and production phases of the oil and gas industry. Large-diameter (4-in. [10-cm]) core provides engineers and geologists with direct means to measure physical properties of reservoir rocks at both the microscopic and properties of reservoir rocks at both the microscopic and macroscopic levels. This information provides engineers with clues to improve their understanding of the reservoir and prediction of its performance. If stored properly, core may prediction of its performance. If stored properly, core may assist in development of the reservoir many years after the well is drilled.

In microlaminated reservoirs, laboratory core analysis is very important because of inherent limitations in wireline log resolution. In these cases, petrophysical information, such as saturation, porosity, and net feet of pay, cannot be calculated from wireline data. Instead, these data must be measured directly from core plugs in the laboratory. Historically, core recovery in these types of reservoirs has not been good (Fig. 1A) using methods designed for firmly consolidated formations. These methods did not achieve satisfactory recovery in unconsolidated sand interbedded with hard shale stringers for two reasons:unconsolidated sand was eroded by mechanical or hydraulic means andshale "jammed" in the core barrel, thereby preventing more core from entering. Changes in coring strategies and equipment have nearly eliminated recovery problems in unconsolidated sand while reducing jams in shale (Fig. 1B). This paper discusses several of these changes and presents ideas for further improvements.

Coring Objectives In Laminated Sequences

The primary objective in coring laminated sand and shale sequences is to obtain continuous core throughout the entire coring interval. Typically, 4-in. [10-cm] -diameter core is obtained in 30-ft [9-m] sections with 81/2-in. [22-cm] bits. The method is time-consuming and relatively expensive because a large percentage of time is required to trip in and out of the hole to remove the core. The obvious solution of using longer barrels has not proved successful because of the inability to prevent or to detect jams reliably. prevent or to detect jams reliably.

Standard Operating Procedures

Operating procedures for obtaining maximum core recovery at minimum cost are generally the same for laminated sequences and homogeneous sands. To avoid barrel jams or core damage, light bit weights and low rotary speeds are used to minimize bending of the relatively limber core barrel. Low flow rates are used to minimize hydraulic erosion of core. After coring, tripping out of the hole should be slow, particularly through tight spots.

Maximizing Recovery in Unconsolidated Sand

Although many operating procedures do not vary from standard practice for hard rock, equipment selection and strategy differ practice for hard rock, equipment selection and strategy differ substantially because unconsolidated sand can be easily washed away by mud flow before or after entering the core barrel. It can also "fluidize" as pressure decreases during pulling out of hole and can be lost in the hole.

Full-Opening and Full-Closing Catcher. Recovery has been improved by use of full-opening and full-closing catchers instead of conventional catchers that must be forced open by the core entering the barrel. Bearing friction initially causes the inner barrel to rotate with the outer barrel. This also rotates conventional catchers relative to the core and may erode weak core before it enters the barrel. Because full-opening catchers are mechanically held in the open position during coring, recovery is improved by removing obstructions to core entering the barrel. The full-closing feature prevents core loss during circulation or pulling out of the hole. Full-opening and frill-closing catchers should be used for weak or unconsolidated formations, but not for highly competent formations that may be too hard to allow closure. Because they are more complex than conventional catchers, the higher risk of failure should be justified.

Overbalance.

Maintaining a minimum mud overbalance above formation pressure has also improved recovery. Laboratory tests and field experience have shown that unconsolidated sand will fluidize when pore pressure is nearly equal to bottomhole mud pressure. With at least 400-psi [2.8-MPa] overbalance, even loose sand grains will enter the, barrel without fluidizing. However, too high an overbalance can cause differential sticking, low rate of penetration (ROP). and excessive core flushing with mud filtrate.

Bit Design.

A third factor affecting recovery is bit design. High ROP is desirable to prevent core erosion and to reduce cost. High ROP is best achieved with large cutters, such as conventional polycrystalline diamond compact (PDC) or thermally stable PDC bits. Conventional PDC bits are best for soft formations without laminations, particularly if overbalance is high, because they can penetrate below the zone of high differential pressure.

Face discharge reduces core erosion compared with throat discharge. Another feature designed to reduce erosion is to place the cutters that first engage the formation ahead of fluid place the cutters that first engage the formation ahead of fluid flow. Recent experience indicates that if full-opening/ full-closing catchers are used with at least 400-psi [2.8-MPa] overbalance, these features are not only unnecessary but sometimes undesirable.

JPT

P. 1544