Long-Term Integrity of Well Cements at Downhole Conditions

Abstract

The objective of plug and abandonment of wells can be described as Љrestoring the cap rockЉ. In that respect, the long-term integrity of the plugging material is crucial. I.e. it is important that the plugging material can resist downhole chemicals and otherwise withstand downhole conditions. In this paper, we have performed ageing tests with cement samples at relevant downhole conditions to determine the long-term integrity of well cement as plugging material. Portland cement samples with and without silica flour as additive have been separately exposed to crude oil, brine and H 2 S (in brine) at 100°C and 500 bar for 1, 3, 6 and 12 months. The long-term integrity of the samples was determined by measuring changes in weight, volume, mechanical strength and permeability, as well as physical appearance.It is seen from the results that the addition of a pozzolan such as silica has a significant impact on the long-term integrity of Portland cement, especially in a corrosive environment such as H 2 S. All the samples were affected by most of the different chemical environments, but the cement samples without silica were considerably more affected than the samples with silica as additive. Furthermore, the exposure to H 2 S in brine resulted in the formation of an unexpected white deposit, which precipitated both inside and outside the samples.

Specification for Permanent Plugging Materials

“…Nonetheless, the definition of long time is a matter of interpretation as an established definition has not been published. Some researchers have selected 1-, 3-, 6-, and 12-month time intervals as the exposure time in their studies [28,30,31]. However, it is recommended to continue for much longer periods, even up to 5 years [2].…”

“…Any degree of expansion may fracture the adjacent formation and any degree of shrinkage may cause microannuli and or debonding. The volume changes of a WBE may diminish the formation radial stresses and if it falls below the formation pore pressure, the risk of uncontrolled fluid flow is increased [31,40]. Therefore, measuring the volume changes of well barrier elements as one of the durability parameters is vital.…”

Types of Permanent Plugging Materials

“…Therefore, as eternal perspective is impossible, use of long-term durability knowledge of intended plugging materials is important. It is necessary to expose plugging materials to downhole chemicals, at downhole conditions, for different time intervals; sometimes up to a few years [22][23][24][25]. Then, the mechanical properties of plugging materials are characterized at different time intervals.…”

“…Thus, different researchers have selected different chemicals with different dosages to study the degradation of oilwell cements at downhole conditions. Vralstad et al [25] aged neat class G cement, at downhole conditions, by exposing the samples to brine, crude oil, and H 2 S dissolved in brine for different Durability of neat class G cement exposed to downhole chemicals [25] time intervals, up to 12 months. They also studied the weight and volume changes of the cement.…”

Long-term durability of rock-based geopolymers aged at downhole conditions for oil well cementing operations

“…The ageing tests with H 2 S required a different experimental setup due to health, safety and environmental (HSE) concerns; therefore these tests were performed at a lower pressure than the crude oil and brine tests. [18] The crude oil and brine exposure tests were performed by placing the samples inside autoclaves filled with the respective fluids. Three parallel samples were tested for each time period mentioned above.…”