Cements with special properties

Well cement. In oil and gas well drilling for sealing the oil, gas and water-bearing strata is made of so-called cementation annulus. These standardized bonded metal tube (so-called body), whose diameter is not less than 5 cm diameter holes injected into the lower hole and is cemented by introducing a cement slurry between the borehole wall and the pipe. The cement mantle used to secure the pipe in the well is pumped through which oil and natural gas, as well as for protection against salt corrosion and deposit water. Grout is pumped from below into the annular space between the pipe and the surrounding rock. Details of the method of deep drilling cementing are described in section 4.5 of the drilling tsementirovanii.Tsementy used in the drilling cementing must meet special requirements. They must work at high temperatures and pressures (currently under technical temperatures up to 250 ° C and pressures up to 1500 bar), should have a low viscosity and pumped at a distance of 20 km completely filling the annular space, and then quickly sets and hardens. Particular importance is attached precisely calibrated time hardening and hardening of cement. Due to the high temperature in the wellbore ordinary cement quickly sets and hardens in the annulus, the cement does not have time to be pumped through and cementing slurries nepolnym.Shvatyvanie be delayed by the addition of lignosulphonate, carboxylate-synthetic polymers (such as AMPS - acrylic acid copolymer) or tartaric acid. The most widespread lignosulfonate retarders which are used for temperatures up to 150 ° C. Suitable inhibitors have on the one hand long enough to delay the cement setting time (in 3-6 hours depending on well depth) and on the other hand, after setting to quickly gain sufficient strength. Finally, the cement should solidify as quickly as possible, and to allow further drilling operations. The minimum compressive strength must be 3.45 MPa (500 psi). This strength is sufficient to withstand the shocks during drilling. Since modern offshore drilling units per day are operating costs by more than S 250,000, the lowest possible latency for cement solidification is economically very important. Used for mixing water and underground water can sometimes contain increased amounts of sulfates in addition to NaCl, CaCl2, MgCl2. Therefore preferably use cements with sulfate-low C3A. To make the cement slurry required properties, such as density, fluidity and control of water loss from the cement slurry used a number of highly specialized additives and impurities, which well cement must show the properties required by the contract. Currently, annual consumption of oil-well cement is about 1 million tons.

Well cement

Designed for cementing oil and gas wells to protect them from the effects of groundwater. They are divided into cement for the "cold" (up to 40? C) and hot "(above 40? C) wells. Thus, the cement for the "cold" wells has increased the amount of C S and C, A, and for the "hot" - With the S and C AF. Start setting oil-well cements for the "cold" wells not less than 2 hours, the "hot" - 1 hour 45 minutes. They maintain high mobility, they have the ability to harden in conditions of high temperatures and pressures developed at great depths in the wells.

 

Well cement is one of the varieties of Portland cement and is designed to isolate the oil and gas wells from the action of groundwater. In accordance with GOST 1581-42 oil-well cement is available in two types:

a) for the "cold" wells;

b) for "hot" wells.

Chemical and mineralogical composition. The chemical composition of well cements do not differ from Portland cement, so that the mineralogical composition of clinker cement grouting on different plants varies quite considerably. Clinker well cement for cold wells are characterized by:

 

a) a high content of tricalcium aluminate (up to 12-13%) when the content of alite about 50%, which provides the required curing speed and increased cement strength in the early stages of solidification;

b) a high content of tricalcium silicate (57- 60%) under a reduced content of NWA (4-7%). In this clinker, if it is sufficiently finely divided, as provided trebuemaya setting speed and high activity of cement in the early stages of hardening. For hot wells, to retard setting time, and store the desired fluidity of cement mortar, the cement is applied with a low content of NWA. In the production of Portland cement clinker tamponazhnogo use the same raw komponenty that for ordinary portlandtsementa.Protsessy hardening and hydration of well cement uskoryayutsya with increasing temperature in the well. At the same time increases the strength of the cement, and the start time setting sokraschaetsya.

 

 

Oilwell cement mixing 50% water, is capable of giving the mobile mass (slurry), which can be pumped into the well by pumps. It is necessary that the hardened cement stone of a dilute pulp has a high initial strength. To adjust the timing setting is added to the cement plaster, the optimal dosage is in the plants selected depending on the mineralogical composition of clinker and cement grinding fineness. The addition of gypsum per S03 must not be more than 3.5%.

Setting time of a cement slurry with 50% of the water should be as follows:

cold wells - the beginning of setting Minimum 3 I and at least 7 hours and 30 minutes after mixing, and the end - not later than 3 hours after the start of the setting;

hot wells - the beginning of the setting on or after 1 h 45 min and no later than 2 hours and 45 minutes after mixing, and the end - not later than 1 hour and 30 minutes after the initial setting. The flexural strength of samples at the age of two days should be:

wells while cold solidification temperature of 22 ° ± ± 2 C for at least 27 kg / cm2;

hot wells at a temperature of solidification of 75 ± ± 3 ° C - not less than 62 kg / cm '.

Fineness of the cement grinding should be such that, when sieving through a sieve № 008 held not less than 25% of the sample weight. The degree of fineness is factory set depending on the obtained indicators spreadability pulp, setting time and the bending strength in a two-day age (as determined in accordance with GOST 1581-42).

Hydration

Hydration reactions are processes in which water is associated with clinker minerals in the hydration products and referred kakgidraty or hydrate phase simultaneously occurs with the cement hydration hardening it; grout or soft cement slurry passes into a solid cement stone. Solidification is divided in practice into two steps of setting and hardening. Immediately after contact of the water and the cement hydrates are formed, which initially leads to a small increase in the strength of the cement, the strength further increases. The attainable strength of the advance linked to the value of a particular cement paste consistency. Initially, this is not an elastic product. Only after a long progressive hydration, which promotes the formation of a solid microstructure ultimately leads to a lasting and solidification, thus the formation of a durable reliable product. During solidification primarily interact with the aluminate phases calcium sulfate, for further hardening the cement stone responsible silicate phases. Cement is a hydraulic binder. Accordingly, the hydration is carried out both in the air and underwater. Cement stone - unlike plaster is waterproof. Products formed during the hydration, practically insoluble in water. The sum of these properties gives Portland cement its unique capabilities as a building material.

The weight ratio of the addition of water to cement is called water-cement ratio (water / cement ratio, W / C).

In the process of the cement hydration and curing various chemical processes occur in parallel and in series. They are often very complicated because the individual clinker phases are not independent of each other, react and can influence each other. In particular, this chemical reaction of hydration and hydrolysis reaction

The resulting solution and crystallization processes in which gel is formed and crystallized hydrated neoplasms (hydrates) are derived from supersaturated solutions as a result topochemical processes.

For a more detailed description of the processes of hydration 4 main clinker phases (alite, belite, aluminate and alyumoferrity) first, they are treated separately. The procedure and reaction rate can be controlled by determining the phase composition of tumors, measuring the heat of hydration and water consumption measurement of the electrical resistance of the cement paste. Particularly important are the reactions hydration aluminate and alite, belite reacts slower but identical Alito. Ferrite has no significant effect.