Simplifying Well Abandonments Using Shale as a Barrier

Abstract

Well abandonment is one of the biggest challenges in the oil and gas industry, both in terms of cost and effort as well as the technical hurdles associated with wellbore isolation for an indefinite term. A mechanism that may be exploited to simplify well abandonments is using natural shale formations for the creation of annular barriers. Currently, uncemented annuli often require casing milling and pulling before abandonment plugs can be set, which necessitates the use of a drilling rig. This is an expensive, time- and labor-intensive process, particularly offshore. However, shale creep may naturally form a barrier behind uncemented casing sections. With a qualified annular shale barrier in place, the well may only require the setting of abandonment plugs within the existing casing string(s), a task that can often be done rigless and with significantly less effort. The work described in this paper presents the results of a rock mechanical investigation into the creep behavior of North Sea shales and their ability to form effective annular barriers. Field core from the Lark-Horda shale was used to conduct dedicated, customized experiments that simulated the behavior of shale confined under downhole effective stress, pressure and temperature conditions to fill in an annular space behind a simulated casing string. Full scale tri-axial rock mechanics equipment was used for testing cylindrical shale samples obtained from well-preserved field core in a set-up that mimicked an uncemented casing section of a well. The deformation behavior of the shale was monitored for days to weeks, and the formation of the annular barrier was characterized using dedicated strain measurements and pressure pulse decay probing of the annular space. The large-scale lab results clearly show that the Lark-Horda shales will form competent low permeability annular barriers when left uncemented, as confirmed using pressure-pulse decay measurements. They also show that experimental conditions influence the rate of barrier formation: higher effective stress, higher temperature and beneficial manipulation of the annular fluid chemistry all have a significant effect. This then opens up the possibility of activating shale formations that do not naturally create barriers by themselves into forming them, e.g. by exposing them to low annular pressure, elevated temperature, different annular fluid chemistry, or a combination. The results are in very good agreement with field observations reported earlier by several North Sea operators.

Acid Treatment as a Way to Reduce Shale Rock Mechanical Strength and to Create a Material Prone to the Formation of Permanent Well Barrier

“…It has been suggested that this phenomenon can potentially be useful in P&A [4,5]. Indeed, it has been confirmed in downscaled laboratory experiments that some shales like e.g., Lark-Horda can efficiently seal off annulus [6,7]. It has been shown that annulus closure may be induced by annulus pressure reduction and may be facilitated either thermally [8] or chemically [6].…”

“…Indeed, it has been confirmed in downscaled laboratory experiments that some shales like e.g., Lark-Horda can efficiently seal off annulus [6,7]. It has been shown that annulus closure may be induced by annulus pressure reduction and may be facilitated either thermally [8] or chemically [6]. Bauer et al used controlled heating to stimulate plastic deformation in shale.…”

“…Their thermo-hydro-mechanically coupled finite-element simulations confirm that heating of a well may result in a strong reduction of the borehole radius and eventually to annulus closure. Van Oort et al have used both heating as well as chemical treatment to accelerate shale creep [6]. They have shown that exposure of Lark-Horda shale to sodium and especially lithium silicate solutions resulted in faster formation of annular barrier compared to the reference test performed with the artificial pore fluid.…”

“…The Lark-Horda shale shown to undergo the chemical acceleration in Van Oort experiment [6] is a weak shale with a Young's modulus of 827.4 MPa and large clay content of 70-73%, thus it is not surprising that by modulating clay swelling shale creep abilities can be engineered. Creep, i.e., time dependent plastic deformation, has been identified as the main shale deformation mechanism [5].…”