High Pressure And High Temperature Research Articles

Synthesis and characterization of G/TiO1.80 bulk composite thermoelectric material under high temperature and high pressure

The primary purpose of this work is to introduce the second phase of graphene (G) into non-stoichiometric TiO1.80 successfully and optimize the thermoelectric properties of this composite material through high pressure and high temperature (HPHT) technology. The purpose of doping Ti powder under high pressure is to create a closed reducing atmosphere to change the ratio of titanium to oxygen in the titanium oxide base. The addition of graphene can considerably improve the electrical properties of the material and reduce its resistivity. An X-ray diffractometer, X-ray photoelectron spectrometer, scanning electron microscope, and transmission electron microscope were used to analyze and characterize the phase structure, chemical bond, micro morphology and crystal morphology of the samples. An abundance of grain boundaries and lattice dislocation defects can inhibit the lattice thermal conductivity. We also tested and analyzed the thermoelectric performance of the high-temperature and high-pressure synthetic samples through a variable temperature system. The variation of the absorption intensity of the ultraviolet UV spectrum with wavelength shows that high pressure can reduce the band gap, which is beneficial to the carrier transition and improves the conductivity of semiconductors. HPHT optimizes both the electrical and the thermal parameters of the sample. At a final sintering pressure of 5.0 GPa, the dimensionless figure of merit (zT) of the bulk composite material G/TiO1.80 was found to be 0.23 at 700 °C.

Interfacial Tension Measured at Nitrogen–Liquid and Liquid–Liquid Interfaces Using Model Microemulsions at High-Pressure and High-Temperature Conditions

The interfacial tensions (IFTs) of two different model microemulsion systems were studied at high-temperature and high-pressure (HTHP) conditions at the nitrogen–brine interface. The experimental scope covers the temperature range 25–100 °C, and a pressure range from atmospheric to 300 bar, more representative of some reservoir conditions. A nonionic ethoxylated alkyl ether based microemulsion (ME1) and a nonionic sugar based microemulsion (ME2) were selected for this study. ME2 was chosen for its tolerance to high salinity and temperature. Favorable interactions between the microemulsions and nitrogen lead to substantial IFT reduction. Low IFT values of 14.36 and 13.1 mN m–1 were measured for ME1 and ME2, respectively, at the highest temperature and pressure settings, far below the IFT value measured for nitrogen–brine with no microemulsion (40.2 mN m–1). Both microemulsions (ME1 and ME2) maintained their IFT lowering capabilities far beyond their respective cloud points. No loss of IFT lowering performance or surfactant phase separation was observed over the studied temperature and pressure ranges. The IFT between 0.2 vol % ME2 and a crude oil was also measured at HTHP conditions. ME2 demonstrated phase stability at the crude oil interface up to 80 °C and retained its IFT lowering performance at even higher temperatures. ME2 reduced interfacial tension at the crude oil–brine interface to values of 0.2–0.5 mN m–1, without any measurable adverse effects of pressure and temperature on the IFT.

Rapid preparation and characterization of pyrite materials under HPHT: A new method

This paper reports the synthesis of bulk pyrite (FeS2) polycrystals at high pressure and high temperature (HPHT) for the first time. The HPHT method markedly improves the speed of synthesis, reducing the time for synthesizing bulk pyrite from several hundred hours to several hours. A unique honeycomb-type structure was introduced into the system by the primary reaction to refine the size of the reactants. On this basis, the sulfur-deficient contamination phase Fe7S8 was effectively eliminated by secondary synthesis. Using Raman spectroscopy and X-ray diffraction (XRD), the crystalline phase of the product was confirmed to be the cubic pyrite phase. The crystal structure and initial crystal parameters of pyrite crystals synthesized under HPHT conditions were obtained for the first time by Rietveld refinement with GASA software. In combination with a scanning electron microscope (SEM) assay, the reaction process was traced, and the significance of secondary synthesis for HPHT was analyzed. Seebeck data showed that as the temperature increased, the sample changed from p-type to an n-type semiconductor and the conductivity increased monotonically, revealing the typical semiconductor characteristics of pyrite. UV reflectance spectra and VSM tests revealed the bandgap width and magnetic properties of pyrite.

A Parametric Study for Radial Cracking in Cement Under Different Loading Events Based on the Stress Intensity Factor

AbstractCement is one of the primary barriers in a wellbore and critical to well integrity. Radial cracking is a pervasive failure mode in cement due to the temperature and pressure variation during drilling, completion, or production. This work presents a comprehensive analysis of radial cracking in cement under various loading events. The proposed model estimates the stress intensity factor and fracture surface displacement as indicators for crack propagation and opening, respectively, through a distributed dislocation technique. Three types of radial cracks, divided by their tips terminating at the casing–cement interface, inside cement, or at the cement–formation interface, are considered. Based on this model, we conduct a parametric study for radial cracking under typical loading events such as steam injection, CO2 injection, and high-pressure and high-temperature (HPHT) drilling. Results indicate that the crack near the casing–cement interface has an increased risk for steam injection and HPHT drilling, while all three types of radial cracks are destructive during CO2 injection. The thermal expansion coefficient of cement is a significant parameter for steam and CO2 injection wells. The fluid pressure and the cement’s thickness are crucial to radial cracking under HPHT conditions. Stiffer cement could promote crack opening for steam injection but prohibit the crack deformation for CO2 injection or HPHT wells. Thicker cement would accelerate radial cracking under the three loading events. These findings are helpful in designing cement to maintain long-term integrity.

Optimization of thermoelectric properties of Al-doped Zn1−xAlxO under high pressure and high temperature

ZnO has excellent electrical properties such as high conductivity (σ) and Seebeck coefficient (S) that are important for its thermoelectric properties and have provided the basis for obtaining improved ZnO-based thermoelectric materials by doping. In particular, the most representative Al-doped ZnO (AZO) has excellent thermoelectric performance. Although the Al doping is beneficial for reducing the lattice thermal conductivity of ZnO, the solubility of Al2O3 in ZnO is limited and excessive doping of Al gives rise to the generation of a large amount of the ZnAl2O4 secondary phase that limits the further improvement of the electrical properties. Thus, it is crucial to develop approaches to reduce thermal conductivity while still maintaining high electrical properties. In this work, high pressure and high temperature (HPHT) synthesis was used to adjust the ZnO optical band gap by modifying the lattice parameters of ZnO, and thereby improving the electrical properties of the sample. It was found that the optimal doping content for the samples synthesized at high pressure is different from that of the samples synthesized at normal pressure, providing a new approach for the further improvement of the thermoelectric properties of ZnO. Additionally, high pressure can promote grain refinement that is conducive to the generation of the multi-scale hierarchical structure of the sample that introduces a variety of phonon scattering mechanisms to reduce the thermal conductivity while maintaining a high power factor. The Zn0.96Al0.04O material obtained by sintering at a pressure of 3 GPa and a temperature of 1053 K for 30 min reached the highest power factor (7.8 µW cm−1 K−2) and the highest zT (0.16) at 973 K.

Effect of deformation of diamond anvil and sample in diamond anvil cell on the thermal conductivity measurement* *Project supported by the National Key Research and Development Program of China (Grant No. 2018YFA0702700) and the National Natural Science Foundation of China (Grant Nos. 11674404 and 11774126).

Studies show that the sample thickness is an important parameter in investigating the thermal transport properties of materials under high-temperature and high-pressure (HTHP) in the diamond anvil cell (DAC) device. However, it is an enormous challenge to measure the sample thickness accurately in the DAC under severe working conditions. In conventional methods, the influence of diamond anvil deformation on the measuring accuracy is ignored. For a high-temperature anvil, the mechanical state of the diamond anvil becomes complex and is different from that under the static condition. At high temperature, the deformation of anvil and sample would be aggravated. In the present study, the finite volume method is applied to simulate the heat transfer mechanism of stable heating DAC through coupling three radiative-conductive heat transfer mechanisms in a high-pressure environment. When the temperature field of the main components is known in DAC, the thermal stress field can be analyzed numerically by the finite element method. The obtained results show that the deformation of anvil will lead to the obvious radial gradient distribution of the sample thickness. If the top and bottom surfaces of the sample are approximated to be flat, it will be fatal to the study of the heat transport properties of the material. Therefore, we study the temperature distribution and thermal conductivity of the sample in the DAC by thermal-solid coupling method under high pressure and stable heating condition.

A novel retarded HCl acid system for HPHT carbonate acidizing applications: Experimental study

AbstractRetarded acid systems for high‐pressure and high‐temperature (HPHT) carbonate reservoirs are the key to provide a deep acid penetration and higher acid efficiency. The current study experimentally evaluates a new retarded HCl acid for HPHT carbonate acidizing applications. The acid retardation mechanism is attained by using a polymeric resin‐based composition that restricts the proton mobility of the acid species. Ten Indiana limestone cores were used with the dimensions of 6 in (0.15 m) length and 1.5 in (0.038 m) diameter and average permeability of 9 mD (0.009 ). Two sets of experiments were conducted, one at 250°F (394 K) and another at 300°F (422 K), with acid injection rates from 0.5–10 cm3/min (8.3E−9 to 1.67E−7 m3/s). The pore volume of acid required to breakthrough (PVBT) was monitored. Effluent samples were collected to track the pH and the dissolved material. Computed tomography (CT) was used to scan the core before and after the experiment to determine the wormhole propagation along with the core. The optimum injection rate at 250 and 300°F (394 and 422 K) were found to be 2.5 and 6 cm3/min (1.67E−8 and 1.25E−7 m3/s) with PVBT of 0.41 and 0.43 PV, respectively. The 3D CT scan showed an effective wormhole profile featuring a single dominant trajectory with few branches. These results are confirmed by calcium ion concentration profiles through inductively coupled plasma (ICP) analysis. The use of the new acid system will provide deep penetration with low injected acid volume. As a result, the stimulation job efficiency will improve.

Comparison of 2507 Duplex and 28 % Cr- Austenitic Stainless Steel Corrosion Behavior for High Pressure and High Temperature (HPHT) in Sour Service Condition with C-ring Experiment

The scarcity of oil and gas resources made High Pressure and High Temperature (HPHT) reservoir attractive to be developed. The sour service environment gives an additional factor in material selection for HPHT reservoir. Austenitic 28 Cr and super duplex stainless steel 2507 (SS 2507) are proposed to be a potential materials candidate for such conditions. C-ring tests were performed to investigate their corrosion behavior, specifically sulfide stress cracking (SSC) and sulfide stress cracking susceptibility. The C-ring tests were done under 2.55 % H2S (31.48 psia) and 50 % CO2 (617.25 psia). The testing was done in static environment conditions. Regardless of good SSC resistance for both materials, different pitting resistance is seen in both materials. The pitting resistance did not follow the general Pitting Resistance Equivalent Number (PREN), since SS 2507 super duplex (PREN > 40) has more pitting density than 28 Cr austenitic stainless steel (PREN < 40). SS 2507 super duplex pit shape tends to be larger but shallower than 28 Cr austenitic stainless steel. 28 Cr austenitic stainless steel has a smaller pit density, yet deeper and isolated.

Filtration and structure of bentonite-β-cyclodextrin polymer microspheres suspensions: Effect of thermal aging time

Effective filtration loss control is a challenge for water-based drilling fluid when drilling high temperature and high pressure (HTHP) formations. After thermal aging at 200 °C for varying times, the static filtration loss controlling capabilities of β-cyclodextrin polymer microspheres (β-CDPs) and synthetic polymer Driscal-D in bentonite suspensions were compared in this study. A comprehensive physicochemical characterization was presented using Fourier transform infrared (FT-IR), X-ray diffraction (XRD), particle size distribution, zeta potential, electrical conductivity and scanning electron microscope (SEM). The results indicated that when exposure to thermal aging of 200 °C, the filtration loss of bentonite suspension increased with the initial 6 h of aging, after that it increased slightly. While for synthetic polymer Driscal-D, the filtration loss increased gradually with thermal aging time until to 36 h. In contrast for β-CDPs, the filtration loss increased firstly and then decreased and maintained relatively stable after thermal treatment for 6 h. High temperature destroyed the hydrated structure of bentonite particles and induced the increase of filtration loss volume. In viewing of Driscal-D, degradation of the polymer in combination with dehydration of bentonite particles led to the gradual losing of suspensions stability. For β-CDPs, after aging of 6 h, the hydrothermal carbonization reaction generated numerous nano and micro carbon spheres and bentonite-carbon sphere composited structures, which fill in the gaps between the larger particles and contribute to reducing the filter cake permeability. This result identified that β-CDPs are superior to synthetic polymer Driscal-D in filtration control when subjected to long term high temperature environments.

Porous single-crystal diamond

Porous diamonds have unique application prospects due to their flexible surface functionalities, high surface-area-to-volume ratio and high self-sharpening ability. Porous diamonds are usually prepared through microwave plasma chemical vapor deposition (MPCVD) techniques or through etching by transition metals, which are expensive and inefficient. Here, we report a simple and efficient method to grow large millimetre-scale porous single-crystal diamonds using high-pressure and high-temperature (HP-HT) techniques. The porous diamond crystal growth is driven by minimizing the surface energy through the attachment of small crystals to a large seed crystal surface in a metal solvent. Industrial-scale production of single-crystal porous diamonds is possible with this discovery.

共焦点ラマン分光法を用いたc-BN含有単結晶ダイヤモンド粒子の結晶性分析とTEM-EELS分析によるヘテロ界面の検討

Several theories have been proposed to synthesize diamond particles from graphite using a metal solvent（also called a catalyst, flux or dissolver）under high-pressure and high-temperature（HPHT）conditions. Many researchers support the “dissolution-precipitation” theory, by which graphitic carbon dissolves in molten metal and subsequently precipitates in diamond form. An alternative is Hosomi's theory: solid state nucleation occurs to form diamond crystals. This study uses confocal Raman spectroscopy to examine the crystallinity of cubic boron nitride （c-BN）contained in a single-crystal diamond matrix synthesized using HPHT method according to Hosomi's theory. We also observed theheterointerface between c-BN and the diamond matrix using transmission electron microscopy to assess the type of bonding present. Constituent elements and the local chemical bonding state were examined using electron energy loss spectroscopy. Results demonstrated that the c-BN and diamond were not bonded directly, but revealed an approx. 5-nm-thick boron-based compound（or amorphous）layer at the interface. The respective concentrations of boron in diamond and carbon in c-BN were below the detection limits, indicating no effect on the crystallinity of either material under the HPHT conditions.

Hardness, magnetic, elastic, and electronic properties of manganese semi-boride synthesized by high pressure and high temperature

Transition metal (TM) interaction induces many fantastic properties in transition metal light element compounds (TMLEs). Here, we report the synthesis of polycrystalline Mn2B specimen with strong Mn–Mn interaction by high pressure and high temperature (HPHT) method. Magnetic characterization gives anti-ferromagnetic and spin-glass behavior with Néel temperature of 43 ​K. Micro-hardness measurement yields an asymptotic Vickers hardness of 14.1 ​GPa which is far higher than manganese metal. To interpret the mechanical and magnetic properties, the chemical bond and electronic structure were investigated by first principle calculation. Although the distance between manganese atoms has been enlarged by inserting boron atoms, anti-ferromagnetic behavior is attributed to the insufficient Mn–Mn distance enlargement. Based on the bonds character, these manganese atoms layers are pinned together by quasi-boron-chains which is contradictory with fully isolated boron atoms configuration. Quasi-covalent boron chains and partial covalent Mn–B bonds attribute to its high hardness. Tuning Mn–Mn interaction is demonstrated as a promising method to modulate d-electron related magnetic and hardness property.

Effects of Size and Surface Properties of Nanodiamonds on the Immunogenicity of Plant-Based H5 Protein of A/H5N1 Virus in Mice.

Nanodiamond (ND) has recently emerged as a potential nanomaterial for nanovaccine development. Here, a plant-based haemagglutinin protein (H5.c2) of A/H5N1 virus was conjugated with detonation NDs (DND) of 3.7 nm in diameter (ND4), and high-pressure and high-temperature (HPHT) oxidative NDs of ~40–70 nm (ND40) and ~100–250 nm (ND100) in diameter. Our results revealed that the surface charge, but not the size of NDs, is crucial to the protein conjugation, as well as the in vitro and in vivo behaviors of H5.c2:ND conjugates. Positively charged ND4 does not effectively form stable conjugates with H5.c2, and has no impact on the immunogenicity of the protein both in vitro and in vivo. In contrast, the negatively oxidized NDs (ND40 and ND100) are excellent protein antigen carriers. When compared to free H5.c2, H5.c2:ND40, and H5.c2:ND100 conjugates are highly immunogenic with hemagglutination titers that are both 16 times higher than that of the free H5.c2 protein. Notably, H5.c2:ND40 and H5.c2:ND100 conjugates induce over 3-folds stronger production of both H5.c2-specific-IgG and neutralizing antibodies against A/H5N1 than free H5.c2 in mice. These findings support the innovative strategy of using negatively oxidized ND particles as novel antigen carriers for vaccine development, while also highlighting the importance of particle characterization before use.

The study on erosion of buckling tubing string in HTHP ultra-deep wells considering fluid–solid​ coupling

As the first barrier of wellbore integrity, tubing string is an important guarantee for safety production. However, with the rapid consumption of oil and gas, the mechanical environment in wellbore is becoming more and more harsh, which poses a challenge to the safe service of tubing string. In order to systematically study the erosion of buckling tubing string in high temperature and high pressure (HTHP) ultra-deep wells, the researches on buckling deformation and erosion of tubing string are carried out gradually in this paper. There are three steps in the research process: (a) The tests including metallographic, mechanical properties and hardness of studied tubing material are carried out, and the test results are the basis of the next numerical simulation; (b) the finite element model (FEM) of whole section tubing sting-packer-casing is established to study the buckling deformation of tubing string due to the comprehensive effect including piston effect, ballooning effect, temperature effect, friction effect and buckling effect; (c) according to the buckling flow passage, FEM of tubing erosion is established based on erosion mode, fluid–solid coupling and impact of sand on each other. Finally, the erosion rule and failure region on tubing are obtained. The researches presented in this paper can provide reference and guidance for predicting erosion and protecting the tubing string.

Long-term thermal performance of oil well cement modified by silica flour with different particle sizes in HTHP environment

The high temperature and high pressure (HTHP) cementing environment results in strength retrogression and structure degradation of oil well cement sheath. Silica flour is commonly incorporated in cement paste as a high-temperature stabilizer to enhance the thermal stability. This work investigated the long-term thermal stability of cement pastes modified by silica flour with different particle sizes. The compressive strengths of cement pastes cured at 80 ℃ × 0.1 MPa and 260 ℃ × 21 MPa for 1, 7, 28, 90, 300 and 400 d were tested. The phase evolution and microstructure development of cement pastes were characterized by XRD, TGA, MIP and SEM. The results showed that the oil well cement pastes incorporated with 200-mesh silica flour had the highest compressive strengths after HTHP curing. The pastes with 1000-mesh silica flour exhibited lower thermal stability, because more calcium-rich hydration products were formed in the matrix after the thermal exposure. The dissolution of silica flour under HTHP conditions was not the main factor controlling the hydration of cement pastes. The 1000-mesh silica flour released more Si4+ than the 200-mesh one. This reduced the diffusion rate of Ca2+, resulting in a decrease in the rate of hydration. This affected the compressive strengths of cement pastes modified by silica flour in HTHP environment.

Emergence of nano silica for oil and gas well cementing: application, challenges, and future scope.

Nanotechnology has opened up a plethora of opportunities and has acquired extreme importance in a myriad of fields to produce enhanced materials. Their special properties make them sustainable for industrial purposes. One of the most crucial processes in the petroleum and geothermal industries is cementing. Various classes of Portland cement are used according to API classifications. The conventional Portland cement fails to perform its function at high pressure and high temperature (HPHT) conditions. Hence, various admixtures are used to improve its properties. HPHT conditions not only have a bad impact on Portland cement by affecting its rheological properties but also reduce its strength, porosity, and permeability. So, additives like nano silica are used to improve its properties. Better compressive strength, low porosity and permeability, higher yield stress, and reduced setting time are some of the major properties that improve by the use of nano silica. This paper discusses in detail the different types of cement, cementing processes, failure of Portland cement, and effect of nano silica as an admixture on the compressive strength, rheology, porosity, and permeability of the cement. Furthermore, the upcoming challenges in cementing are discussed along with future potential in this field.

Corrosion damage behaviors of rubber O-rings under simulated acid fracturing conditions

Abstract To explore the damage behavior of O-ring in acid environment, a high-temperature and high-pressure (HTHP) autoclave was used to simulate the service environment of O-ring, and then 168h corrosion test of hydrogenated nitrile butadiene rubber (HNBR) and fluororubber (FM) O-rings were carried out. The corrosion damage behaviors of two kinds of rubber O-rings in the acidizing fluid were studied through determining their tensile strength, elongation at break, hardness, permanent compressive deformation, tensile fracture morphology and sealing property. The results showed that the cross-sectional area and the compression permanent deformation increased, the tensile strength and hardness decreased when the HNBR and FM O-rings under the free state were subjected to acid corrosion. The elongation at break of HNBR decreased, and that of FM rubber increased greatly. Similar with free state, the HNBR and FM O-rings under sealed state also presented the same variation trend. The decrease in the reliability of the O-rings under the sealed state was less significant than that in the free state. In the test, tensile fractures were mostly brittle fractures, HNBR and FM O-rings had obvious corrosion damages such as deformation and swelling. The results could provide a technical basis for the selection of sealing materials, tool optimization design, and construction work in oil and gas fields.

Based on self-made shale formation simulated artificial cores to evaluate water-based drilling fluids plugging effect technology

Plugging technology is one of the leading problems in shale gas drilling engineering, as it causes wellbore stability, shale formation collapse, and so on. In order to solve these problems, in this paper, we committed to preparing artificial cores whose original permeability was 1.16×10−4 mD, which could be used to simulate the shale formation, and established a sealing evaluation method to assess the effect of plugging technology by measuring permeability reduction rate K'. Verified the good reproducibility of the artificial cores through a lot of high temperature and high pressure (HTHP) filtration experiments. The Scanning electron microscopy (SEM) was used to prove that the permeability reduction rate is a scientific method to evaluate the sealing effect.

Identification and Prediction of Allo-Source Overpressure Caused by Vertical Transfer: Example from an HTHP Gas Reservoir in the Ledong Slope in the Yinggehai Basin

The Yinggehai Basin is a typical high temperature and high pressure (HTHP) gas-bearing basin. The pressure coefficient exceeds 2.2 in deeply-buried Miocene reservoirs in the Ledong Slope, a nondiapir zone in the Yinggehai Basin. Determining the overpressure mechanisms and predicting the pore pressure are key issues for natural gas exploration and development in the Ledong Slope. In this paper, overpressure mechanisms were investigated according to the analysis of vertical effective stress-logging responses and geological evaluations, and the pore pressure was predicted using the Bowers method. The loading-unloading crossplots indicated that the overpressure that existed in reservoirs mainly consists of two types: neighbor-source and allo-source overpressure. The neighbor-source overpressure is mainly caused by the pressure transmission from the adjacent mudstone to the reservoir, with a pressure coefficient less than 1.5 ~ 1.6. The high-magnitude overpressure points with pressure coefficients greater than 1.6 show a typical unloading response, indicating elevated sandstone pressures rather than in situ mudstone pressures, which are most likely to be generated by overpressure vertical transfer. The high-magnitude overpressure fluid generated by the high mature ultradeep buried N1s source rock migrated to the shallower reservoirs via hidden faults/microfractures, which led to the vertical transfer of overpressure. Vertically transferred overpressure was generated at 1.5 ~0.2 Ma, which is beneficial for the preservation of overpressure in lenticular sandbodies. The estimated pore pressure by the Bowers method is in good agreement with the measured pressure and provides a meaningful reference for predrilling pressure prediction in nondiapir or diapir zones in the Yinggehai Basin.

Failure and mitigation study of packer in the deepwater HTHP gas well considering the temperature-pressure effect during well completion test

During the well completion test of deepwater high temperature and high pressure (HTHP) gas well, the force on the packer changes dramatically, leading to the packer failure and serious consequences. Take consideration of the temperature-pressure effect, the packer force calculation model and the failure judgement flow diagram of packer are established based on the theory of tubing mechanics, the law of momentum conservation, and the liquid PVT properties to evaluate the reliability of packer. Dimensionless formulations to obtain the axial force and the pressure difference are proposed, and ten influencing and controllable factors are chosen to the sensitivity analysis. The results show that seven factors have great influence on the axial force and the pressure difference. Finally, two mitigation methods, installing slip joint on the tubing string and injecting nitrogen gas in the annulus, are studied for axial force and pressure difference, and the concept and calculation method of axial force safety factor are put forward. The mitigation effect of these two methods are all obvious. This study can provide theoretical guidance for packer protection during well completion test of deepwater HTHP gas well, thus can contribute to the development of natural gas and even the mitigation of greenhouse effect.